Accept header parameters in RFC 2231 format.

This should fix facebook/tornado#868. Someone should test whether this works as intended with Python 2, though. In gagern/urllib3@e11e03628f8b82021a74ba7f5b39ce9ad9011382 I had to encode the input from unicode to bytes for this to work. Not sure why, though, and also not sure whether this might be handled in some other location. Python 3 accepts this modification without issues.