If-Modified-Since disabled on collection endpoints.

Please see the discussion at #334
(https://github.com/nicolaiarocci/eve/issues/334) for the reasoning
behind this decision. Closes #334.
Merge branch 'fix-weak-date' of github.com:superdesk/eve into superdesk-fix-weak-date
Move RFC1123_DATE_FORMAT to __init__.py

So we can eventually use it elsewhere as needed.
weak_date always returns a RFC-1123 date.
Merge branch 'superdesk-fix-weak-date' into develop
Add support for custom HATEOAS

Updated the code in order to don't overwrite '_self' and 'self' if already exists in doc.