sf_addLoadMoreHandler: crash about sfpullrefresh

[nealsun]

fix:

- (void)setOwner:(UIScrollView *)owner
{
    _owner = owner;
    ...

    static dispatch_once_t once_token;
    dispatch_once(&once_token,  ^{
        SEL orignSelector = @selector(willMoveToSuperview:);
        SEL extendedSelector = @selector(sf_willMoveToSuperview:);
        Method originalMethod = class_getInstanceMethod([_owner class], orignSelector);
        Method extendedMethod = class_getInstanceMethod([_owner class], extendedSelector);

        if (class_addMethod([_owner class], orignSelector,  method_getImplementation(extendedMethod),  method_getTypeEncoding(extendedMethod))) {
            class_replaceMethod([_owner class], extendedSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, extendedMethod);
        }
    });

    ...
}