When dispatching a webhook that contained non-ASCII content, the urlopen call raised an exception

that reported an encoding error, complaining that some characters are not encodable with the ascii

encoding.
This issue occurs because the urlopen implementation concatenates the given URL, headers, and body

using string concatenation, and, if the resulting string is a unicode string, encodes it implicitly

as ASCII before sending it to the server.
This change encodes all data passed to urlopen as binary strings, so that the result of string

concatenation is also a binary string, thus necessitating no implicit encoding. While writing tests

for this issue, an issue with silently swallowed assertions in the test _urlopen was fixed.

Added 4 unit tests for non-ASCII custom, JSON, XML and Form-Data content.

Checked locally that payloads are sent without issue.

Added check that all data sent to _test_dispatch is made of binary strings.