ios: enable camera switching (react-native-webrtc#837)

If the devices has been requested with the parameter `deviceId` like
this `navigator.mediaDevices.getUserMedia({video: {deviceId: 'n'}})`
the method `_switchCamera` will not work. Setting the `_deviceId` to `NULL`
in the `switchCamera` method will allow the change.

Author: alejo-lapix

ios/RCTWebRTC/VideoCaptureController.m

@@ -116,6 +116,7 @@ -(void)stopCapture {
 
 -(void)switchCamera {
     _usingFrontCamera = !_usingFrontCamera;
+    _deviceId = NULL;
 
     [self startCapture];
 }