Java 8: TEI header generation creates invalid namespace declaration [was #16730]

When generating TEI corpus documents, the TEI header is created by transforming an XML representation of the respective TextGrid metadata documents using an XSLT 1.0 stylesheet. The transformation is explicitely run using the JRE's built-in Xalan (cf. aggregator:commit:53f6b22e ).

While this worked correctly in Java 7, with Java 8, the generated TEI documents contains an invalid namespace declaration: <teiHeader xmlns:xmlns="http://www.tei-c.org/ns/1.0" xmlns:md="http://textgrid.info/namespaces/metadata/core/2010">, which, of course, triggers a XML parser error in later transformations.