networkaddress: fix parsing of uri with @ after authority

Make sure that the @ sign is inside the authority part before attempting
to parse the userinfo. We do this by checking if the @ sign comes before
any of the possible authority delimiters.
Add unit test to verify parsing of ftp://ftp.gnome.org/start?foo=bar@baz

https://bugzilla.gnome.org/show_bug.cgi?id=726040