fixes failing test

@talex5 - you mind having a look at this? Your last PR broke a test [1] and this is my blind attempt at fixing it.


[1] https://github.com/rgrinberg/ocaml-cohttp/blob/fix_tests/lib_test/test_parser.ml#L170
more of a nit, but could also do a `match !remaining with 0 -> Final_chunk ...` to continue the same style.