Add "Allow" header to HTTP 405 error.

By RFC7231 (http://tools.ietf.org/html/rfc7231#section-6.5.5)
405 status must generate Allow header with list of resource's supported
methods.
Merge remote-tracking branch 'upstream/master'
Add test for checking Allow header on 405 http error
Add python 3.2+ support for 405 error Allow header test

In python 3.2+ unittest.TestCase.assertItemsEqual method was replaced by
assertCountEqual method.
See http://bugs.python.org/issue17866.
Merge pull request #294 from singulared/master

Add "Allow" header to HTTP 405 error.