Make Style/RedundantSelf aware of arguments of a block

[Envek]

Currently, Rubocop complains about redundant self when using a block with argument name same with receiver method, like this:

  %w[draft preview moderation approved rejected].each do |state|
    define_method "#{state}?" do
      self.state == state
    end
  end

Autocorrection fixes this code to state == state effectively breaking it.

This PR removes offense for arguments of a block with names equal to receiver method name.

---

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists). Not exists
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• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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  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). Not required

[Drenmi]

Nice catch, and nice PR. 🙂 Could you please add another example to the documentation and run rake generate_cops_documentation?

[Envek]

I found that it doesn't handle nested blocks well (as in the first comment), so I changed implementation (and test) to deal with it.

Added example to documentation.

[bbatsov]

I'm curious how is this different than passing a param from a method? Aren't block params lvars just as well?

[bbatsov]

In particular I see that we already had this in the code:

def on_blockarg(node)
    on_argument(node)
end

[Envek]

The main problem is that previous add_scope method cleared local variables for each nested scope that is fine for methods, because they can't be nested, but blocks usually are nested deeply.

Rebased on top of current master.