Return 204 if there is no content

If after serializing the response we do not have a body or headers (for
example after a delete), we should return a 204 code.

ooi/tests/middleware/test_compute_controller.py

@@ -104,7 +104,7 @@ class TestComputeController(test_middleware.TestMiddleware):
         expected_result = ""
         self.assertContentType(resp)
         self.assertExpectedResult(expected_result, resp)
-        self.assertEqual(200, resp.status_code)
+        self.assertEqual(204, resp.status_code)
 
     def test_list_vms_one_vm(self):
         tenant = fakes.tenants["foo"]

ooi/tests/test_wsgi.py

@@ -68,7 +68,7 @@ class TestMiddleware(base.TestCase):
     def test_show(self):
         result = webob.Request.blank("/foos/stop",
                                      method="GET").get_response(self.app)
-        self.assertEqual(200, result.status_code)
+        self.assertEqual(204, result.status_code)
         self.assertEqual("", result.text)
 
     def test_post(self):

ooi/wsgi/__init__.py

@@ -302,6 +302,11 @@ class ResponseObject(object):
             if body:
                 response.body = body
 
+            # 204 should be used if there is no content
+            if (not (headers or body) and
+                    response.status_int in [200, 201, 202]):
+                response.status_int = 204
+
         return response
 
     @property