+load method in the category should be guarantee execute onlyonce

[robinli08]

• (void)load
  {
  Method delegateOriginalMethod = class_getInstanceMethod([self class], @selector(setDelegate:));
  Method delegateSwizzledMethod = class_getInstanceMethod([self class], @selector(vi_setDelegate:));

  method_exchangeImplementations(delegateOriginalMethod, delegateSwizzledMethod);
  }