Updated zend.http.headers.rst for header 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
Merge pull request #1128 from whymarrh/fix/zend.http.headers.rst

Updated zend.http.headers.rst to reflect header-specific methods
Merge branch 'feature/1128' into develop

Close #1128