Return a {@link ContentHandler} interface to provide SAX input to.Through the returned object the document to be serailized, as a series of SAX events, can be provided to the serialzier. If the serializer does not support the {@link ContentHandler}interface, it will return null.
In principle only one of asDOMSerializer() or asContentHander() should be called.
@return A {@link ContentHandler} interface into this serializer,or null if the serializer is not SAX 2 capable
@throws IOException An I/O exception occured