use session.deserialize to unpack message for rate limiting

[minrk]

rather than hardcoding json.loads, which fails on Python 3.5

Messages should never be deserialized by any means other than the Session API.

[jdfreder]

Ah interesting, I was doing this earlier but kept running into an error because I tried to deserialize the same message twice (duplicate signatures). I didn't think to change the base method like you do here.

[minrk]

If need be, you could use session.unpack to unpack a single frame (I vacillated on whether I should do that here). But that's still putting an assumption about the wire format in code that shouldn't have it.