When inlining a DataHandler as Base64, OMTextImpl doesn't stream the data

When converting a DataHandler to Base64, OMTextImpl#writeOutput ends up calling OMTextImpl#getText, meaning that Axiom will construct a single String from the complete DataHandler content. For large contents this will inevitably lead to out of memory errors. The correct strategy would be to stream the data, i.e. to do the conversion in chunks and write several CHARACTER events to the XmlStreamWriter.

Both LLOM's OMTextImpl and DOOM's TextImpl are affected by this problem.