Possibly a fix for the debian issue. See #425.
Fix for #427: Preconditions checking for http delete.

And also refactored the DELETE tests a bit.
Bumped vobject 3.x test to 3.2
Merge branch '1.7' into 1.8

Conflicts:
	tests/Sabre/DAV/Mock/Collection.php
	tests/Sabre/DAV/ServerSimpleTest.php
Adding PHP namespaces.