Precaution for UIImage ImageWithURLRequest

The following code does not give a warning in xcode..Quite strange.

It took me some time to find the problem. This commit makes sure the URL is always wrapped in a NSURLRequest.

    [myImageView setImageWithURLRequest:[NSURL URLWithString:@"http://example.com/image.png"]
                       placeholderImage:[UIImage imageNamed:@"placeholder"]
                                success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
                                    //do something
                                } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
                                    //do nothing
                                }];