Use HEAD on Container panel ajax update

Change-Id: I9d9e56951e6fe0b8b9cd5242be43c0abb9f9ec4c
Closes-bug: 1257238
Increase Horizon base unit test suite.

Add unittests for templatetags, decorators, filters and common utils.
Complement tests merged on commit #258e9b465314d33e669b77b52cec63b2c8aca370

Change-Id: I3f4486815a11c22d6bbcaa014b8e632b43d3be89
Merge "Use HEAD on Container panel ajax update"
Merge "Increase Horizon base unit test suite."