Don't crash when header is value of None

Before commit 7d88d14d (http://git.io/hhJWdQ) swifclient used to accept
header value with the type None :

{'Headers': None}

It would just be happy with it with those None headers and not process
them, reinstate the old behavior.

Closes-Bug: 1256441
Change-Id: Ic7f80c38ef2ce9ef1687ed0d6d65521f2e754905
Merge "Don't crash when header is value of None"