Added proof for #439.
Fixed the implementation of PATCH so it makes sense again. #439.
Updated changelog
Merge branch '1.7' into 1.8

Conflicts:
	lib/Sabre/DAV/FSExt/File.php
	lib/Sabre/DAV/PartialUpdate/Plugin.php
More bugfixes for #439.
Merge branch '1.7' into 1.8

Conflicts:
	lib/Sabre/DAV/PartialUpdate/Plugin.php