generateDS assumes, that XSD types may be used for several different
tags. Although this is true in general, making the default tag name for
a type identical to its tag name doesn't work well for PageXML (as it is
usually the tag name but with "Type" appended).

In nested tags, the generateDS export function sets the tag name
correctly, but when not specifying it in the root export call, the root
element gets a wrong tag name. Therefore the default value is now
changed to the correct tag.

This may be done for the other types in the future, but as it is only
required for exporting a partial xml file and easily worked around by
specifying the name_ parameter, it is not a priority.