ApplicationError fails when mixing python2 and python3

I've encountered a crash when using a Python2 router with a Python3 client.

Indeed, when the router send a autobahn.wamp.message.Error containing kwargs the client crashes:

Traceback (most recent call last):
  File "/home/emmanuel/projects/autobahn_sync/.tox/py34/lib/python3.4/site-packages/autobahn/wamp/websocket.py", line 90, in onMessage
    self._session.onMessage(msg)
  File "/home/emmanuel/projects/autobahn_sync/.tox/py34/lib/python3.4/site-packages/autobahn_sync/session.py", line 32, in onMessage
    return super(_AsyncSession, self).onMessage(msg)
  File "/home/emmanuel/projects/autobahn_sync/.tox/py34/lib/python3.4/site-packages/autobahn/wamp/protocol.py", line 921, in onMessage
    txaio.reject(on_reply, self._exception_from_message(msg))
  File "/home/emmanuel/projects/autobahn_sync/.tox/py34/lib/python3.4/site-packages/autobahn/wamp/protocol.py", line 248, in _exception_from_message
    if msg.kwargs:
TypeError: __init__() keywords must be strings

The trouble is this kwargs's keys are made of bytes (i.e. python2's str) but python3 wants unicode (python3's str) to use them as function parameters

(Pdb) msg.kwargs
{b'message': 'event history for the given subscription is not available or enabled'}

The WAMP spec (https://tools.ietf.org/html/draft-oberstet-hybi-tavendo-wamp-02#section-5.2) only define the utf8-encoded string, I guess then everything should be in UTF-8.
Given there is no trouble when using Python3 everywhere, that mean the data's encoding depends of the sender which I believe is a bug both at serialization (should only send data in one encoding) and deserialization (should only accept data in one encoding) time.

Start at autobahn/wamp/protocol.py in _exception_from_message and trace the message path from autobahn/wamp/websocket.py's onMessage, then inspect how kwargs are serialized and deserialized between Python 2 and Python 3. Reproduce the mixed-version router/client case with the shown bytes key; done means an Error containing kwargs no longer raises the TypeError and the encoding behavior is consistent with the WAMP string requirement.