Take Track write Lock in NewRTPSender

Track is locking for reading when actually writing
is taking place

README.md

@@ -131,6 +131,7 @@ Check out the **[contributing wiki](https://github.com/pion/webrtc/wiki/Contribu
 * [David Hamilton](https://github.com/dihamilton)
 * [Ilya Mayorov](https://github.com/faroyam)
 * [Patrick Lange](https://github.com/langep)
+* [cyannuk](https://github.com/cyannuk)
 
 ### License
 MIT License - see [LICENSE](LICENSE) for full text

rtpsender.go

@@ -33,8 +33,8 @@ func (api *API) NewRTPSender(track *Track, transport *DTLSTransport) (*RTPSender
 		return nil, fmt.Errorf("DTLSTransport must not be nil")
 	}
 
-	track.mu.RLock()
-	defer track.mu.RUnlock()
+	track.mu.Lock()
+	defer track.mu.Unlock()
 	if track.receiver != nil {
 		return nil, fmt.Errorf("RTPSender can not be constructed with remote track")
 	}