Added Undefined offset test for reading XmlDataSet

[Sicaine]

If you define n columns and add n + 1 rows it will generate an undefined offset error.

Example .xml file:

<?xml version="1.0" encoding="UTF-8"?>

id 1 1

Therefore a check is necessary. Because $rowElement->children()->count() didn't work for me properly it was necessary to test the index in the foreach instead of testing it beforehand with something like 'if ($rowElement->children()->count() >= count($tableInstanceColumns) {}

[Sicaine]

I changed it in the second commit. Is that patch okay or do i need to do anything else?

It was quit strange to figure out how to fix a patch.

[elazar]

Merged. Thanks!