RMLStreamer fails when encounters xmlns prefix

[larisa1995]

When setting an XML file with xmlns prefix as input, RMLStreamer outputs an error "org.w3c.dom.DOMException: NAMESPACE_ERR: An attempt is made to create or change an object in a way which is incorrect with regard to namespaces." and deletes an output file.

If it helps, there was a similar error in RMLmapper [1].

[1] RMLio/rmlmapper-java#154