Update AddDataToCustomerSection.php

[WouterSteen]

When you add gtm data on the section data with another plugin, this function overrides the gtm data. With a recursive function this will be added next to existing data.

[jissereitsma]

I'm going to merge this, but it might be that this actually leads into new issues: array_merge_recursive sometimes turns strings into arrays if multiple similar nodes exist. But we'll see at that moment.