Close the mirror client socket on rejection

If the mirror attempt connects ok, but is rejected (say, for reporting
the wrong size), the client socket needs to be closed.  The destination
end can't close its socket and accept another connection attempt unless
it does.

tests/acceptance/fakes/dest/hello_wrong_size.rb

@@ -21,4 +21,9 @@ client.write_hello( :size => :wrong )
 
 t.join
 
+# Now check that the source closed the first socket (yes, this was an
+# actual bug)
+
+fail "Didn't close socket" unless client.disconnected?
+
 exit 0