soup: Use soup_session_new() to avoid deprecation warning

soup: Use soup_session_new() to avoid deprecation warning

There are a ton of libsoup/GLib-related "leaks" that we can't whitelist
and with leak detective active there is a delay that interestingly doesn't
happen with soup_session_sync_new(), so tests failed with a timeout (actually
they hung due to the lock in the fetcher manager).
On Travis, the curl plugin is used for the tests, so that's not an issue
there (and without LD the tests complete quickly and successfully).