BUG/MEDIUM: cache: bad computation of the remaining size

The cache was not setting the hdrs_len to zero when we are called
in the http_forward_data with headers + body.

The consequence is to always try to store a size - the size of headers,
during the calls to http_forward_data even when it has already forwarded
the headers.

Thanks to Cyril Bonté for reporting this bug.

Must be backported to 1.8.
1 file changed