Fix chunked transfer-encoding

If a chunk header line fell on a buffer boundary just right, parsing
would fail.

Test response data is at https://gist.github.com/retupmoca/11147476 - the initial socket read gets the '0' character, but not any of the '\r\n' stuff after it, so the chunk parsing fails without this patch. I'm not sure what the best way to put this into a .t file is.