Add uuid field to AnimationClip.toJSON

When converting the scene to JSON all objects except animation clips have uuid fields included. This change adds the uuid to the JSON object for AnimationClips.

src/animation/AnimationClip.js

@@ -59,7 +59,8 @@ Object.assign( AnimationClip, {
 
 			'name': clip.name,
 			'duration': clip.duration,
-			'tracks': tracks
+			'tracks': tracks,
+			'uuid': clip.uuid
 
 		};