HttpParser and multiple concatenated HTTP responses

Hi!

If my understanding is correct HttpParser.execute returns the number of bytes consumed, hence it is possible to parse multiple concatenated HTTP responses by checking is_message_complete() and starting a new parser from the same very position the last one stopped.

However the observed behavior is very different: execute() happily consumes the whole string, the subsequent responses text ends up in the message body and _clen_rest is negative.

Start by reproducing the concatenated-response case around HttpParser.execute() and is_message_complete(), then inspect how _clen_rest is updated when a response ends. Done means the parser reports the first response boundary without placing subsequent response text in the body, so another parser can start at the returned position.

Written by the indexing model from the issue text.