BUG/MINOR: http: properly handle all 1xx informational responses

BUG/MINOR: http: properly handle all 1xx informational responses

Only 100 was considered informational instead of all 1xx. This can be
a problem when facing a 102 ("progress") or with the upcoming 103 for
early hints. Let's properly handle all 1xx now, leaving a special case
for 101 which is used for the upgrade.

This fix should be backported to 1.7, 1.6 and 1.5. In 1.4 the code is
different but the backport should be made there as well.