Issue #482: PropFind::set() hard adds properties during ALLPROPS

Here's my crack at solving Issue #482 myself. Works when I test with `cadaver` and verify the response with a tcpdump. I'm not too confident in my understanding of the property code at large though, so please review.

Most of the diff are whitespace changes since I removed an indentation level; running a `git diff -w HEAD~` gives a better idea of the changes made.