If we promise wsgi.input_terminated, we better get it right - or not at all.
* chunked encoding on HTTP <= 1.1
* chunked not last transfer coding
* multiple chinked codings
* any unknown codings (yes, this too! because we do not detect unusual syntax that is still chunked)
* empty coding (plausibly harmless, but not see in real life anyway - refused, for the moment)
