SplitHTTP: Do not produce too large upload (#3691)

transport/pipe/writer.go

@@ -19,6 +19,10 @@ func (w *Writer) Close() error {
 	return w.pipe.Close()
 }
 
+func (w *Writer) Len() int32 {
+    return w.pipe.Len()
+}
+
 // Interrupt implements common.Interruptible.
 func (w *Writer) Interrupt() {
 	w.pipe.Interrupt()