Fix #7070. Correctly use animation frame duration when it is set.

Author: zekeatchan

src/animations/Animation.js

@@ -361,8 +361,8 @@ var Animation = new Class({
     {
         //  When is the first update due?
         state.accumulator = 0;
-
-        state.nextTick = state.msPerFrame || state.currentFrame.duration;
+        
+        state.nextTick = state.frameRate === state.currentAnim.frameRate ? state.currentFrame.duration || state.msPerFrame : state.msPerFrame;
     },
 
     /**
@@ -515,7 +515,7 @@ var Animation = new Class({
     {
         state.accumulator -= state.nextTick;
 
-        state.nextTick = state.msPerFrame || state.currentFrame.duration;
+        state.nextTick = state.frameRate === state.currentAnim.frameRate ? state.currentFrame.duration || state.msPerFrame : state.msPerFrame;
     },
 
     /**