Fix for #422 on the 1.7.

Also took the opportunity to reorganize some of the unittests a little
bit.
Merge branch '1.7' into 1.8

Conflicts:
	composer.json
	tests/Sabre/DAV/ServerFinderBlockTest.php
	tests/Sabre/DAV/ServerSimpleTest.php
	tests/Sabre/DAVServerTest.php
Fixed #422 for the 1.8 branch.
Merge branch '1.8'

Conflicts:
	ChangeLog
	lib/Sabre/DAV/Server.php
	tests/Sabre/DAV/ServerFinderBlockTest.php
	tests/Sabre/DAV/ServerSimpleTest.php
	tests/Sabre/DAVServerTest.php