[BUGFIX] Fix wrong test in method registerProviderExtensionKey

According to the method definition $extensionKey is given as string.
Assuming, the logic is to search an extension key from an array of
extensions, the test must be fixed solving a warning on the FE.
Merge pull request #136 from fudriot/patch1

[BUGFIX] Fix wrong test in method registerProviderExtensionKey
[BUGFIX] Fix Warning if configuration wizard is empty

It may happen configuration wizard is empty. In this case, method
xml2array will return a debug string telling something went wrong. Since,
later in the code, it is assumed the output of xml2array is an array, a PHP
warning will be raised on the FE.
Merge pull request #137 from dreipunktnull/master

[TASK] Code cleaning
Merge pull request #135 from fudriot/master

[BUGFIX] Fix Warning if configuration wizard is empty
Merge pull request #138 from dreipunktnull/master

[BUGFIX] Fix inheritance in DataViewHelper