Fix wrong place of isQuaternion property

src/math/Quaternion.js

@@ -19,8 +19,6 @@ function Quaternion( x, y, z, w ) {
 
 Object.assign( Quaternion, {
 
-	isQuaternion: true,
-
 	slerp: function ( qa, qb, qm, t ) {
 
 		return qm.copy( qa ).slerp( qb, t );
@@ -165,6 +163,8 @@ Object.defineProperties( Quaternion.prototype, {
 
 Object.assign( Quaternion.prototype, {
 
+	isQuaternion: true,
+
 	set: function ( x, y, z, w ) {
 
 		this._x = x;