Add HTTP header passing test to pick header for downloading request

While developing my app using SDWebImage, I meet this problem: I have to download an image with OAuth 2 access token in HTTP header, so I use `-[SDWebImageDownloader setValue:forHTTPHeaderField:]` to set `Authorization` header, but doing this will include access token to _all_ the image downloading request which will expose access token. This PR could solve this problem by letting developer to choose which header to include for image URL. Sample code is shown below:

``` objc
SDWebImageManager.sharedManager.imageDownloader.HTTPHeadersPassingTest =
^BOOL(NSURL *url, NSString *field, NSString *value) {
    return [field isEqualToString:@"Authorization"] ? [url.host isEqual:@"foo.bar.com"] : YES;
};
```