Server.handleCall: Run methods when the ctx is already canceled

...and let the method implementation check the context itself.

This solves an occasional hang (observed in capnproto#318) in TestRecvCancel,
which involves a method implementation that waits for its context to be
canceled and then closes a channel. Without this patch, if the context
is cancelled early enough for handleCall to see it, it won't run the
method at all, and so the channel will never be closed, causing the test
to hang on a receive on that channel.

server/server.go

@@ -203,10 +203,7 @@ func (srv *Server) handleCalls(ctx context.Context) {
 func (srv *Server) handleCall(ctx context.Context, c *Call) {
 	defer srv.wg.Done()
 
-	err := ctx.Err()
-	if err == nil {
-		err = c.method.Impl(ctx, c)
-	}
+	err := c.method.Impl(ctx, c)
 
 	c.recv.ReleaseArgs()
 	if err == nil {