b'...' is not JSON serializable

[nathan-osman]

When passing a dict as an attachment to Connection.send(), there is no check whether the content value is a unicode or binary value. If a binary value is passed, the following exception is raised:

File "connection.py", line 80, in send
    'attachments': list(self._process_attachments(attachments)),
...
File "/usr/lib/python3.5/json/encoder.py", line 180, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: b'...' is not JSON serializable

The correct behavior would be for Connection._process_attachments() to detect this, encode the value with base64, and set encoded to True in the JSON payload to Hectane.