Bugfix: problems with cookies and related functions

Bugfix: problems with cookies and related functions

The get_max_age() and get_expires() functions should support returning NULL.
The set_expires() function parameter tests are incorrect.

Try to return 0 for max age and expires.
- My documentation states that if both expires and max age are 0, then a session cookie is used.
- Therefore, avoid that until I have to double check the standards and my code.

I missed a few underscore to dash conversions in the HTTP header processing code.

Added a 'response_encoding' parameter so that I could quickly test and confirm if the encoding is causing any problems.