Fix getting header in redirect processing

The code for processing an HTTP redirect included an incorrect method of
getting the Location header from an HTTPResponse.  It needs to use the
getheader() method on the response, instead.  This patch fixes that and
includes a unit test that covers this code path.

Change-Id: I0952fabad581b020dee07bdc4007b55b47c906aa
Closes-Bug: #1231524

glanceclient/common/http.py

@@ -233,7 +233,8 @@ class HTTPClient(object):
             raise exc.from_response(resp, body_str)
         elif resp.status in (301, 302, 305):
             # Redirected. Reissue the request to the new location.
-            return self._http_request(resp['location'], method, **kwargs)
+            return self._http_request(resp.getheader('location', None), method,
+                                      **kwargs)
         elif resp.status == 300:
             raise exc.from_response(resp)