Enhance scroll preventDefault logic.

Now scroll preventDefault works more like default browser scroll.
When the scroll leaches top, bottom, left and right end, don't
prevent default scrolling. For sure, when the content size is
smaller than the container size, also don't prevent default scrolling.

src/perfect-scrollbar.js

@@ -166,6 +166,25 @@
 
         // bind handlers
         var bindMouseWheelHandler = function() {
+            var shouldPreventDefault = function(deltaX, deltaY) {
+                var scrollTop = $this.scrollTop();
+                if(scrollTop == 0 && deltaY > 0 && deltaX == 0) {
+                    return false;
+                }
+                else if(scrollTop >= content_height - container_height && deltaY < 0 && deltaX == 0) {
+                    return false;
+                }
+
+                var scrollLeft = $this.scrollLeft();
+                if(scrollLeft == 0 && deltaX < 0 && deltaY == 0) {
+                    return false;
+                }
+                else if(scrollLeft >= content_width - container_width && deltaX > 0 && deltaY == 0) {
+                    return false;
+                }
+                return true;
+            };
+
             $this.mousewheel(function(e, delta, deltaX, deltaY) {
                 $this.scrollTop($this.scrollTop() - (deltaY * 10));
                 $this.scrollLeft($this.scrollLeft() + (deltaX * 10));
@@ -173,7 +192,7 @@
                 // update bar position
                 updateBarSizeAndPosition();
 
-                if(content_height > container_height || content_width > container_width) {
+                if(shouldPreventDefault(deltaX, deltaY)) {
                     e.preventDefault();
                 }
             });