Send identity_headers through the wire

Change I09f70eee3e2777f52ce040296015d41649c2586a, introduced a bug where
the identity_headers are not added to the request headers anymore
causing the former to be completely ignored and useless.

This patch fixes that issue by restoring the previous code. A new test
has been added to avoid regressions.

Closes-bug: #1394965
Change-Id: I1b1633636448398cf3f41217f1d671b43ebd9946

glanceclient/common/http.py

@@ -157,6 +157,10 @@ class HTTPClient(object):
         headers = kwargs.pop("headers", {})
         headers = headers and copy.deepcopy(headers) or {}
 
+        if self.identity_headers:
+            for k, v in six.iteritems(self.identity_headers):
+                headers.setdefault(k, v)
+
         # Default Content-Type is octet-stream
         content_type = headers.get('Content-Type', 'application/octet-stream')