fix calculation of the length of appended STREAM frames

For the last STREAM frame we omit the Length field. When packing STREAM
frames, we need to account for this byte saving when calculating the
length of the payload.

framer.go

@@ -114,7 +114,11 @@ func (f *framerI) AppendStreamFrames(frames []wire.Frame, maxLen protocol.ByteCo
 	}
 	f.mutex.Unlock()
 	if frameAdded {
-		frames[len(frames)-1].(*wire.StreamFrame).DataLenPresent = false
+		lastFrame := frames[len(frames)-1].(*wire.StreamFrame)
+		lastFrameLen := lastFrame.Length(f.version)
+		// acount for the smaller size of the last STREAM frame
+		lastFrame.DataLenPresent = false
+		length += lastFrame.Length(f.version) - lastFrameLen
 	}
 	return frames, length
 }