don't trigger update from checkPassword, it is already  called by userExists, this is enough.

do not write to database when the value is the same

add test cases. also split test classes in a file each, looks like only the first class is being executed

also appconfig shall not write to database if the value is unchanged

* test case added to avoid adding them later

using array_key_exists() instead of isset() - required because in case the value is null isset is returning false

This patch was automatically generated as part of the following inspection:
https://scrutinizer-ci.com/g/owncloud/core/inspections/cdfecc4e-a37e-4233-8025-f0d7252a8720

Enabled analysis tools:
 - PHP Analyzer
 - JSHint
 - PHP Copy/Paste Detector
 - PHP PDepend

Issue #7111

On SQLite the app order can be arbitrary and cause strange bugs.
On MySQL, the app order seems to be always alphabetical.

This fix enforces alphabetical order to make sure that all environments
behave the same and to reduce bugs related to app loading order.

Fixes #6442

Conflicts:
	lib/private/vcategories.php

Implemented unittest for OC\AppConfig

This is NoSpaceAfterComma

Fixed a bug in appconfig, where an initially empty set value led to endless duplicates of this app/key pair.

Path by Elias Probst <mail@eliasprobst.eu>