BUG/MINOR: filters: Count HTTP headers as filtered data but don't forward them

BUG/MINOR: filters: Count HTTP headers as filtered data but don't forward them

In flt_analyze_http_headers() HTTP analyzer, we must not forward systematically
the headers. We must only count them as filtered data (ie. increment the offset
of the right size). It is the http_payload callback responsibility to decide to
forward headers or not by forwarding at least 1 byte of payload. And there is
always at least 1 byte of payload to forward, the EOM block.

This patch depends on following commits:

 * MINOR: filters: Forward data only if the last filter forwards something
 * MINOR: http-htx: Add a function to retrieve the headers size of an HTX message

This patch must be backported with commits above as far as 1.9. In HAProxy 2.0
and 1.9, the patch must be adapted because of the legacy HTTP code.