Fix and cleanup

[lpaulsen93]

This fixes array operations and includes some code cleanup.

[Klap-in]

@lpaulsen93 looks good. You have test the changes?

[lpaulsen93]

@Klap-in I did a short test and that seemed to work fine for me. But I don't use the plugin myself.

[Klap-in]

Should this address #9?

[lpaulsen93]

Could be but I am not sure so I asked for testing in #9.

[Klap-in]

@lpaulsen93

The unit tests show these errors here (locally) (unit test are not enabled at github..)

Failed asserting that null matches expected 'bar'.
Expected :bar
Actual   :null
<Click to see difference>

 /../dokuwiki/lib/plugins/meta/_test/rendering.test.php:29
 

Failed asserting that two strings are equal.
Expected :'My abstract'
Actual   :'Persistent description'
<Click to see difference>

 /../dokuwiki/lib/plugins/meta/_test/rendering.test.php:45