content size checks are not valid for LOCK

refs https://github.com/owncloud/core/issues/9832#issuecomment-55282725

@avoine please test

@icewind1991 @PVince81 @bantu please review

@karlitschek @craigpg should be backported to stable7

How to test:

1. connect with a webdav client (e.g. cadaver)
2. perform a LOCK on an not existing file - in this case the file will be created which causes the original error