BUG/MINOR: compression: Test payload size only if content-length is specified

BUG/MINOR: compression: Test payload size only if content-length is specified

When a minimum size is defined to performe the comression, the message
payload size is tested. To do so, information from the HTX message a used to
determine the message length. However it is performed regardless the payload
length is fully known or not. Concretely, the test must on be performed when
a content-length value was speficied or when the message was fully received
(EOM flag set). Otherwise, we are unable to really determine the real
payload length.

Because of this bug, compression may be skipped for a large chunked message
because the first chunks received are too small. But this does not mean the
whole message is small.

This patch must be backported to 3.2.