Add HEAD api response for test s3 server BucketHandler

Current nova test s3 server lacks HEAD for BucketHandler. And it becomes
an issue for Boto after version 2.25.0 [1], which changes underlying
implementation of get_bucket method from GET to HEAD request.

This change fixes this gap by adding HEAD response to test s3 server.
And also added cases for testing bucket existence per Boto document
suggestion[2], which applies for both Boto prior to or after version
2.25.0

[1] http://docs.pythonboto.org/en/latest/releasenotes/v2.25.0.html
[2] http://docs.pythonboto.org/en/latest/s3_tut.html#accessing-a-bucket

Change-Id: If992efa40f7f36d337d1b9b1f52859aa0ae51874
Closes-bug: #1277790
Fix a couple of unit test typos

There were two cases in rpcapi unit tests where it was checking method instead
of rpc_method.  Fix up the typo and the reason why the typo didn't matter
before.  Also fix two places where rpcapi code had a return for a cast(), which
is now caught by the unit tests.

Closes-bug: #1272518
Change-Id: Ib5e3a002f6b0f7eb8dee8bd84bd53dbf817017f7
Merge "Fix a couple of unit test typos"