Allows user to override default SDWebImageDownloaderOperation

Sets a subclass of `SDWebImageDownloaderOperation` as the default `NSOperation` to be used each time SDWebImage constructs a request operation to download an image.

On `setOperationClass:`, pass the subclass of `SDWebImageDownloaderOperation` to set as the default `NSOperation` class. Passing `nil` will revert to `SDWebImageDownloaderOperation`.