Change ForceSSLHandler#secure? to use == instead of regex

It's more clear and 52x faster in majority of scenarios (matches) and
24x faster on redirects (mismatches)

It also does remove case insensitive searching, but it discussion on the
PR #1662 stated this should be okay functionality to change

Benchmark:

```crystal
require "benchmark"

puts "Matches"
Benchmark.ips do |x|
  x.report("String ==") do
    "https" == "https"
  end

  x.report("!! String =~ /https/i") do
    !!("https" =~ /https/i)
  end
end

puts "\n\nMismatches"
Benchmark.ips do |x|
  x.report("String == mismatch") do
    "http" == "https"
  end

  x.report("!! String =~ /https/i mismatch") do
    !!("http" =~ /https/i)
  end
end
```

```plaintext
➜  tmp crystal build --release downcase-includes-vs-regex.cr && ./downcase-includes-vs-regex
Matches
            String == 897.53M (  1.11ns) (± 6.76%)   0.0B/op        fastest
!! String =~ /https/i  17.13M ( 58.36ns) (±16.32%)  16.0B/op  52.38× slower

Mismatches
            String == mismatch 881.32M (  1.13ns) (± 8.78%)   0.0B/op        fastest
!! String =~ /https/i mismatch  36.42M ( 27.46ns) (± 1.47%)  16.0B/op  24.20× slower
```

src/lucky/force_ssl_handler.cr

@@ -48,7 +48,7 @@ class Lucky::ForceSSLHandler
   end
 
   private def secure?(context) : Bool
-    !!(context.request.headers["X-Forwarded-Proto"]? =~ /https/i)
+    context.request.headers["X-Forwarded-Proto"]? == "https"
   end
 
   private def redirect_to_secure_version(context : HTTP::Server::Context)