Add test cases for send-template.
Expand template_content dict into Mandrill's name/value array.
Don't send template_content as "None" if missing.
Clean up some variable names in the backend.
Add DjrillBackendMockAPITestCase.assert_mandrill_called; use it in representative backend test cases.
(Also make get_api_call_data work with various ways of calling requests.post.)
