Include the sheen parameter while serializing

and deserializing material

src/loaders/MaterialLoader.js

@@ -61,6 +61,7 @@ MaterialLoader.prototype = Object.assign( Object.create( Loader.prototype ), {
 		if ( json.color !== undefined ) material.color.setHex( json.color );
 		if ( json.roughness !== undefined ) material.roughness = json.roughness;
 		if ( json.metalness !== undefined ) material.metalness = json.metalness;
+		if ( json.sheen !== undefined ) material.sheen = (new Color()).setHex( json.sheen );
 		if ( json.emissive !== undefined ) material.emissive.setHex( json.emissive );
 		if ( json.specular !== undefined ) material.specular.setHex( json.specular );
 		if ( json.shininess !== undefined ) material.shininess = json.shininess;

src/materials/Material.js

@@ -169,6 +169,7 @@ Material.prototype = Object.assign( Object.create( EventDispatcher.prototype ),
 		if ( this.roughness !== undefined ) data.roughness = this.roughness;
 		if ( this.metalness !== undefined ) data.metalness = this.metalness;
 
+		if ( this.sheen && this.sheen.isColor ) data.sheen = this.sheen.getHex();
 		if ( this.emissive && this.emissive.isColor ) data.emissive = this.emissive.getHex();
 		if ( this.emissiveIntensity && this.emissiveIntensity !== 1 ) data.emissiveIntensity = this.emissiveIntensity;