BUG/MINOR: proto-htx: Consider a XFER_LEN message as chunked by default

BUG/MINOR: proto-htx: Consider a XFER_LEN message as chunked by default

An HTX message with a known body length is now considered by default as
chunked. It means the header "content-length" must be found to consider it as a
non-chunked message. Before, it was the reverse, the message was considered with
a content length by default. But it is a bug for HTTP/2 messages. There is no
chunked transfer encoding in HTTP/2 but internally messages without content
length are considered as chunked. It eases HTTP/1 <-> HTTP/2 conversions.

This patch must be backported to 1.9.