XML transform task does not allow to produce true text or csv file

[Emmanuel]

Currently when using a XML transform task, even when specifying and output of txt (not text btw) or csv, the transform actually always generate a xml header line: <?xml version="1.0"?>

This forbids the creation of csv of text files with specific formats, and files needs to be re-processed outside of the workflow to remove that line...

We should ideally support true output as "text"...