soup-message-io: fix wrong-Content-Length logic

soup-message-io: fix wrong-Content-Length logic

Previously we just ignored Content-Length if the server specified
"Connection: close", which does the right thing if the specified
Content-Length is too large, but fails if the Content-Length is
correct but the server "forgets" to close the connection. Fix this so
that we always stop reading once we get to the expected
Content-Length, but we also cope with the connection closing before
that point.

https://bugzilla.gnome.org/show_bug.cgi?id=611481