RRDP XML strings review

RRDP XML strings review

Not much to report on this one. It seemed the code was casting string
types back and forth too carelessly, but it turns out its assumptions
are agreeable.

In particular, I got scared by what appeared to be a missing printable
ASCII validation. But no; libxml2 takes care of it, thanks to the XML
grammar.

So what was really missing was a comment explaining the logic, so I
don't have to dig it up again. And some unit tests to preserve the
guarantees.

There are other validations I'm not so sure about, but I decided to
leave them out of the scope of this commit.