fix auth_secret identify option on python3

[ploxiln]

The root issue is that on python3 there are conflicting type assertions for auth_secret:

class AsyncConn(...):
    def __init__(...):
         assert isinstance(auth_secret, string_types + (None.__class__,))
...
    def _on_response_continue(...):
        self.send(protocol.auth(self.auth_secret))

# in protocol.py ...

def auth(data):
    return _command(AUTH, data)

def _command(cmd, body, *params):
    assert isinstance(body, bytes_types), 'body must be a bytestring'

in #202, @ericb proposed just changing the initial type assertion to allow bytes_types instead of string_types. I'm fine with that, but I think it might be more "natural" to allow a string for auth_secret as well, so I've come up with my own variation using to_bytes.

(pushing just the test first to demonstrate failures, then the fix ...)

[ploxiln]

test isn't working at all on travis, dunno why, hang on a sec ...

[ploxiln]

Ah - test doesn't pass on python2.7 where I would expect it to, because of from __future__ import unicode_literals, and the failure is impossible to understand because the exception is hidden in a different context and not captured, annoyingly enough. Anyway the fix should fix it.

[ploxiln]

cc @jehiah