Assign the default value of 60 FPS to the optional frameRate parameter, when ommited.

This patch tries to prevent an issue in the tween timeline data generation
where an `undefined` value causes an unhandled type coercion when the program
calculate how many frames will be generated for a given `TweenData` timeline.

Author: rblopes

src/tween/Tween.js

@@ -835,6 +835,7 @@ Phaser.Tween.prototype = {
             return null;
         }
 
+        if (typeof frameRate === 'undefined') { frameRate = 60; }
         if (typeof data === 'undefined') { data = []; }
 
         //  Populate the tween data