Fix detecting HTML in mobipocket document
Now also detect the following:
<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<html xmlns=\"http://www.w3.org/1999/xhtml\" lang=\"en-US\">
Now also detect the following:
<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<html xmlns=\"http://www.w3.org/1999/xhtml\" lang=\"en-US\">