check how t:insert[@mode='html'] deals with namespaces

[mpo]

When we insert html neko will clean it, but I'm unsure about which namespaces it will be injecting. We should take care that the xhtml namespace gets properly added (or allow to control that via some additional t:insert attribute) to make sure that later sax processing of the template output can correctly space the included html.

This is a follow up task on completing the bare bones functionality of html inserting in #335

NOTE: haven't done any tests yet (those should at least be added) - it might just work already (uhuh)