Fix gallery thumbnail generation

@icewind1991 It seems the create method does not append the username that has been determined in the constructor. The other two commits are cosmetics.

@karlitschek @DeepDiver1975 This might be a reason for thumbnails beeing generated over and over.

may fix #1008 by correcting the path where the created thumbnail is stored.
related PR: https://github.com/owncloud/apps/pull/1189 - should be closed