Add response header OpenAPI spec generation

[jdmurphy]

Add support to expose response headers object

• Available only with desc DSL block syntax under success and failure keys.
• Consumer is responsible for documenting headers

[coveralls]

Coverage increased (+0.001%) to 99.529% when pulling 98fcf7b on jdmurphy:response_headers into a73657b on ruby-grape:master.

[LeFnord]

@jdmurphy thanks for it, but I'm a bit confused, your title says something abvout response example, but your code handles headers …

background: there exists a key headers for the describe method, for documenting it
did your change take the same line?

… and sorry, mörged your second PR rivght now

[jdmurphy]

@LeFnord, sorry that was some bad copy/pasting because the feature is similar in scope to @gamartin's addition of response examples.

When I used the headers key in the describe block, it showed up as a parameter and not in the response area. It is possible I was not using it correctly/there was something I missed when I added it; if so please let me know.

[LeFnord]

@jdmurphy … yeap, you are right, success and failures will be used for documenting the response and the headers key as kind as parameter for the request

[LeFnord]

thanks for your contributions @jdmurphy