Lazy join over allow headers

[ericproulx]

Hi,

While profiling my app, I found this :

266 "GET, HEAD"
266 /Users/eproulx/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/grape1.3.0/lib/grape/api/instance.rb:230

60  "POST"
60  /Users/eproulx/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/grape1.3.0/lib/grape/api/instance.rb:230

30  "PATCH"
30  /Users/eproulx/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/grape1.3.0/lib/grape/api/instance.rb:230

22  "GET, POST, HEAD"
22  /Users/eproulx/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/grape1.3.0/lib/grape/api/instance.rb:230

20  "GET, PATCH, HEAD"
20  /Users/eproulx/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/grape1.3.0/lib/grape/api/instance.rb:230

I figured that joining allowed headers while not actually requesting OPTIONS is a small waste of memory. Instead, we could join them only when needed.

I also replaced string literal 'OPTIONS' by the Http::Headers::OPTIONS

Thanks

[grape-bot]

	1 Warning
⚠️	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.

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