Prevent Layout/SpaceAroundBlockParameters from breaking on lambda

[pocke]

Layout/SpaceAroundKeyword cop crashes on lambda literal that does not have parens.

-> a { puts a }

$ rubocop --debug
An error occurred while Layout/SpaceAroundBlockParameters cop was inspecting /tmp/tmp.OXikUmBqDF/test.rb:1:0.

1 error occurred:
An error occurred while Layout/SpaceAroundBlockParameters cop was inspecting /tmp/tmp.OXikUmBqDF/test.rb:1:0.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
Mention the following information in the issue report:
0.49.0 (using Parser 2.4.0.0, running on ruby 2.4.1 x86_64-linux)
For /tmp/tmp.OXikUmBqDF: configuration from /home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/config/default.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/config/enabled.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/config/disabled.yml
Inspecting 1 file
Scanning /tmp/tmp.OXikUmBqDF/test.rb
undefined method `end_pos' for nil:NilClass
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/layout/space_around_block_parameters.rb:54:in `check_no_space_style_inside_pipes'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/layout/space_around_block_parameters.rb:44:in `check_inside_pipes'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/layout/space_around_block_parameters.rb:26:in `on_block'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/commissioner.rb:44:in `block (2 levels) in on_block'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/commissioner.rb:106:in `with_cop_error_handling'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/commissioner.rb:43:in `block in on_block'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/commissioner.rb:42:in `each'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/commissioner.rb:42:in `on_block'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/ast/traversal.rb:12:in `walk'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/commissioner.rb:60:in `investigate'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/team.rb:112:in `investigate'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/team.rb:100:in `offenses'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cop/team.rb:42:in `inspect_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:258:in `inspect_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:205:in `block in do_inspection_loop'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:237:in `block in iterate_until_no_changes'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:230:in `loop'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:230:in `iterate_until_no_changes'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:201:in `do_inspection_loop'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:111:in `block in file_offenses'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:121:in `file_offense_cache'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:109:in `file_offenses'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:100:in `process_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:78:in `block in each_inspected_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:75:in `each'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:75:in `reduce'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:75:in `each_inspected_file'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:67:in `inspect_files'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/runner.rb:39:in `run'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cli.rb:82:in `execute_runner'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/lib/rubocop/cli.rb:28:in `run'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/bin/rubocop:13:in `block in <top (required)>'
/usr/lib/ruby/2.4.0/benchmark.rb:308:in `realtime'
/home/pocke/.gem/ruby/2.4.0/gems/rubocop-0.49.0/bin/rubocop:12:in `<top (required)>'
/home/pocke/.gem/ruby/2.4.0/bin/rubocop:22:in `load'
/home/pocke/.gem/ruby/2.4.0/bin/rubocop:22:in `<main>'
C

Offenses:

test.rb:1:1: C: Layout/SpaceInLambdaLiteral: Do not use spaces between -> and opening brace in lambda literals
-> a { puts a }
^^^^^^^^^^^^^^^
test.rb:1:4: C: Style/StabbyLambdaParentheses: Wrap stabby lambda arguments with parentheses.
-> a { puts a }
   ^

1 file inspected, 2 offenses detected
Finished in 0.07601038400025573 seconds

This change fixes the problem, and adds specs for lambda literals.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).