DataHandler.writeTo should have built-in support for byte arrays and Strings

Users of JavaMail often create DataHandlers with an object and an unknown
(no DataContentHandler) MIME type. The object is often a simple type such
as a byte array or a String. JavaMail fails to "serialize" these types
because DataHandler doesn't know how to convert these objects to a byte stream.
DataHandler should do the obvious "conversion" for byte arrays and Strings.