soup-message-io: fix retry-after-unexpected-connection-close

soup-message-io: fix retry-after-unexpected-connection-close

When sending a request on a previously-used connection, we have to
deal with the possibility of the server deciding to time out the
connection right as we start sending data (which sounds like a crazy
race condition, but is in fact pretty much standard behavior). This
got broken in the connection/session reorg earlier in the year. Fix
it.

Also, add a test to misc-test for this.

Based on patch from Sergio Villar.

https://bugzilla.gnome.org/show_bug.cgi?id=631525