Creates a File containing with the XML representation for the given WireFeed.
If the feed encoding is not NULL, it will be used in the XML prolog encoding attribute. The platform default charset encoding is used to write the feed to the file. It is the responsibility of the developer to ensure the feed encoding is set to the platform charset encoding.
NOTE: This method delages to the 'Document WireFeedOutput#outputJDom(WireFeed)'.
@param feed Abstract feed to create XML representation from. The type of the WireFeed mustmatch the type given to the FeedOuptut constructor.
@param file the file where to write the XML representation for the given WireFeed.
@throws IllegalArgumentException thrown if the feed type of the WireFeedOutput and WireFeeddon't match.
@throws IOException thrown if there was some problem writing to the File.
@throws FeedException thrown if the XML representation for the feed could not be created.