Fix: `respond_to?` takes 2 arguments

The `respond_to?` method was optimized for less object creation, unfortunately I also mangled the method signature. This commit reverts the changes introduced in #737 to `BodyProxy#respond_to?` and adds tests.

See: https://github.com/rack/rack/pull/737/files#r18359323

cc/ @raggi
Merge pull request #739 from schneems/schneems/fix-respond_to

Fix: `respond_to?` takes 2 arguments