Fix std.experimental.xml.writer

[Computermatronic]

std.experimental.xml.writer.writeDOM does not compile when used, due to a missing foreach aggregate. I've added one, and fixed all of the things that prevent dub test from completing. There's a unittest failure in domimpl.d(2536), not to sure what's with that, not to familiar with the codebase.

[rikkimax]

Looks like a unittest is failing, fix that first and I'm happy to merge once CI is green.