Merge pull request #1125 from whymarrh/fix/zend.http.headers.rst

Update for new Zend\Http\Header classes
Merge branch 'feature/1125' into develop

Close #1125
Updated zend.http.headers.rst for headers APIs

Updated zend.http.headers.rst to reflect the header-specific methods exposed
by each of the different header classes, and abstract base classes.

Also updated:

- Accept*
- RetryAfter
- SetCookie