Previously, we only accepted iterables of strings like 'Header: Value'.
Now, we'll also accept lists of tuples like ('Header', 'Value') as well
as dictionaries like {'Header': 'Value'}.
This should be more intuitive for application developers, who are
already used to being able to pass dicts or lists of tuples to libraries
like requests.
Change-Id: I93ed2f1e8305f0168b7a4bd90c205b04730da836
a1e2bcde4a