The code in Slack (which Mattermost uses as well) to send messages had a

string type problem. A Unicode string was being passed as the payload

body to urlopen(), which wasn't valid on Python 3. While the exception

was caught, it meant that no messages could be sent to either service.
This change fixes the string type, and updates all the relevant unit

tests to perform checks on the string types, headers, and payload

contents.
The unit test updates also simplify the tests, introducing common

utility functions to each suite that performs all the common tests.

Unit tests pass on Python 2 and 3.