Error in rest_client when checking for response body

When executing Tempest with resize tests enabled I get two stack-traces (both for json and xml):

======================================================================
ERROR: The server's RAM and disk space should be modified to that of
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tempest/tempest/tests/compute/servers/test_server_actions.py", line 153, in test_resize_server_confirm
    self.client.confirm_resize(self.server_id)
  File "/home/tempest/tempest/services/compute/xml/servers_client.py", line 300, in confirm_resize
    str(Document(conf)), self.headers)
  File "/home/tempest/tempest/common/rest_client.py", line 171, in post
    return self.request('POST', url, headers, body)
  File "/home/tempest/tempest/common/rest_client.py", line 217, in request
    raise exceptions.ResponseWithNonEmptyBody(status=resp.status)
ResponseWithNonEmptyBody: RFC Violation! Response with 204 HTTP Status Code MUST NOT have a body

And the culprits are these bodies are:

JSON:
{"confirmResize": null}

XML:
<?xml version="1.0" encoding="UTF-8"?>
<confirmResize />

By looking closely, it looks like rest_client.py line 217 is checking for the wrong body.

I am starting to think that we need unit tests to check for the sanity of the functional tests ;)