Fix locateIdentifiersByType to correctly aggregate source

[AydinHassan]

Hi,

It seems the aggregate source locator was not working as += ignores keys that already exist in left hand array:

The + operator returns the right-hand array appended to the left-hand array; for keys that exist in both arrays, the elements from the left-hand array will be used, and the matching elements from the right-hand array will be ignored.

refs: http://php.net/manual/en/language.operators.array.php

[asgrim]

LGTM 👍

[asgrim]

Thanks @AydinHassan