Add missing test coverage for XMLResponseSerializer

Currently we have no direct coverage of this, so add some tests
prior to refactoring in a subsequent patch.

Change-Id: I20242e60b84f9265cdc7afef2deabee8235e287c
Related-Bug: #1291079
Refactor to separate serializers from wsgi controller

Remove the serializers from heat.common.wsgi, so we break the
circular import which happens if you want to import
heat.api.aws.exceptions to do a determination based on exception
type, which is required to avoid the faultwrap exception disguise
which is not applicable to the CFN API.

Partial-Bug: #1291079

Change-Id: I7498d78f8ec6098b28fb183eaaa04aa81fced3eb