Fix part file ignoring

This solves https://github.com/owncloud/mirall/issues/465 and in addition supresses post write hooks for ignored files (currently *.part and .htaccess by default)

.part files are completely ignored and never make it into the cache, as @dragotin suggested in https://github.com/owncloud/core/pull/2528 

@schiesbn please test, the PHP notice should no longer occur
@karlitschek @icewind1991 @MTGap please review.

I'll backport this to OC5 now