[#28367] Cache does not cache header information

This pull request tries to fix the problem mentioned here:

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=28367

It adds the caching of header data (from JResponse::getHeaders()) and removes the use of 'Workarounds' for the cache controller 'page' because I don't think that this is necessary for caching the complete output of a page (there were some problems with that, too).

If your tests are successful I will try to send this also to the Joomla! platform.

This pull request replaces #236