让 UISearchBar 支持空搜索

让 UISearchBar 支持空搜索，当没有输入的时候，search 按钮一样可以点击
- (void)searchBarTextDidBeginEditing:(UISearchBar *) searchBar
{
    UITextField *searchBarTextField = nil;
    NSArray *views = ([[[UIDevice currentDevice] systemVersion] floatValue] < 7.0) ? searchBar.subviews : [[searchBar.subviews objectAtIndex:0] subviews];
    for (UIView *subview in views)
    {
        if ([subview isKindOfClass:[UITextField class]])
        {
            searchBarTextField = (UITextField *)subview;
            break;
        }
    }
    searchBarTextField.enablesReturnKeyAutomatically = NO;
}