fix server panic trying to send on stream as client disconnects #1111 (#1115)

Author: edrex

transport/handler_server.go

@@ -179,11 +179,18 @@ func (a strAddr) String() string { return string(a) }
 
 // do runs fn in the ServeHTTP goroutine.
 func (ht *serverHandlerTransport) do(fn func()) error {
+// Avoid a panic writing to closed channel. Imperfect but maybe good enough.
 select {
-case ht.writes <- fn:
-return nil
 case <-ht.closedCh:
 return ErrConnClosing
+default:
+select {
+case ht.writes <- fn:
+return nil
+case <-ht.closedCh:
+return ErrConnClosing
+}
+
 }
 }