Fixes handling of a split frame message

[lunswor]

Fixes UnicodeDecodeError when receiving messages split across reads, seen occasionally on the cricket stream. Happens when a multi byte character lands at the buffer size split.

For example, a multi-byte character such as an em dash

frame: {"a":"b—c"}\r\n
recv 1:  b'{"a":"b\xe2'
recv 2: b'\x80\x94c"}\r\n'

Currently untested, I still need to test against the socket.

[liampauling]

I had been wondering why this happened for a few years now 😂