BUG/MEDIUM: mux-h2: Don't send CANCEL on shutw when response length is unkown

BUG/MEDIUM: mux-h2: Don't send CANCEL on shutw when response length is unkown

Since commit 473e0e54 ("BUG/MINOR: mux-h2: send a CANCEL instead of ES on
truncated writes"), a CANCEL may be reported when the response length is
unkown. It happens for H1 reponses without "Content-lenght" or
"Transfer-encoding" header.

Indeed, in this case, the end of the reponse is detected when the server
connection is closed. On the fontend side, the H2 multiplexer handles this
event as an abort and sensd a RST_STREAM frame with CANCEL error code.

The issue is not with the above commit but with the commit 4877045f1
("MINOR: mux-h2: make streams know if they need to send more data"). The
H2_SF_MORE_HTX_DATA flag must only be set if the payload length can be
determined.

This patch should fix the issue #1992. It must be backported to 2.7.