BUG/MINOR: h1: Fail to parse empty transfer coding names

BUG/MINOR: h1: Fail to parse empty transfer coding names

Empty transfer coding names, inside a comma-separated list, are already
rejected. But it is only by chance. Today, it is detected as an unknown
coding names (not "chunked" concretly). Then, it is handled by the H1
multiplexer as an error and a 422-Unprocessable-Content response is
returned.

So, the error is properly detected in this case, but it is not accurate. A
400-bad-request response must be returned instead. Then, it is better to
catch the error during the header parsing. It is the purpose of this patch.

This patch should be backported as far as 2.6.