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It's definitely the case that an AssertionError shouldn't be
raised.  It was my intention that the assert never fail.  Do
you know which part of the assert fails?  Perhaps you could
change the assert to print the values of _line_consumed and
_line_left and run some more tests.
I can't figure out how this goes wrong.

Also, do you know when the server is sending an HTTP/0.9
response?  I wonder if there's a bug somewhere else and the
client is guessing the wrong protocol version for the response.