Ignore NoneType when encoding headers

Some generated header values may in fact be None. Trying to encode None
causes the client to fail with an exception and cannot be worked around
by the user.

Change-Id: I638b1fba0ef9a07d726445d8c2cdd774140f5b83
Closes-bug: 1415935

glanceclient/common/http.py

@@ -147,7 +147,7 @@ class HTTPClient(object):
                   names and values
         """
         return dict((encodeutils.safe_encode(h), encodeutils.safe_encode(v))
-                    for h, v in six.iteritems(headers))
+                    for h, v in six.iteritems(headers) if v is not None)
 
     def _request(self, method, url, **kwargs):
         """Send an http request with the specified characteristics.