Fixes issue #2429 - use a stable sort when sorting Accept and Accept-Language headers.

Accept-Language headers, similar to those for Accept headers, and added
a position paramter to the array maps produced so as to take into
account original position when sorting items in order of preference.
Updated CHttpeRequestTest to reflect and validate these changes.
Updated CHANGELOG to include this bug fix.

This PR is to replace PR #2431, which I could not merge properly.