test-utils: bump up debug_level when running under "make check"

Since automake's "make check" rule now logs the output of tests to a
log file, we might as well use maximum debugging, so that if something
fails, we have plenty of explanation why.

(We still parallelize parallelizable tests by default though, meaning
the output is not especially useful for those ones.)
soup-server: clarify soup_server_[un]pause_message() documentation

If a SoupMessage which hasn’t yet started to be received is passed
into soup_server_[un]pause_message(), an assertion will fail in
soup-message-io.c because the message doesn’t yet have any I/O data.

Document that passing such SoupMessages into those methods is naughty
and shouldn’t be done.

https://bugzilla.gnome.org/show_bug.cgi?id=710602
soup-message: minor fixups to soup_message_set_request/response_body()

g_warn_if_fail() if the Content-Type is invalid, and impove the
annotations.

https://bugzilla.gnome.org/show_bug.cgi?id=686766
session-test: fix to work with the dummy TLS backend

do_property_tests() apparently didn't work with the dummy TLS backend,
half because of a glib bug, and half because of a session-test bug.
This fixes the session-test bug; the glib bug is fixed in glib master.
SoupMessage: add :request-body-data and :response-body-data properties

Make it easier to use the request and response bodies from
introspection by providing accessors to get them as GBytes.

https://bugzilla.gnome.org/show_bug.cgi?id=704105