Pass a dict when no extra_headers is specified to upload.py.

It is because upload.py doesn't use sane default parameter.

TBR=dpranke@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7075029

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@87012 0039d316-1c4b-4281-b951-d872f2087c98

rietveld.py

@@ -48,7 +48,7 @@ class Rietveld(object):
       self.rpc_server = upload.HttpRpcServer(
             self.url,
             get_creds,
-            extra_headers=extra_headers)
+            extra_headers=extra_headers or {})
     else:
       self.rpc_server = upload.GetRpcServer(url, email)
     self._xsrf_token = None