[11.x] Fix only number as session key will result in numbered session keys

[Katalam]

Will fix #51599

This will happen because array_merge works like

If the input arrays have the same string keys, then the later value for that key will overwrite the previous one. If, however, the arrays contain numeric keys, the later value will not overwrite the original value, but will be appended. Values in the input arrays with numeric keys will be renumbered with incrementing keys starting from zero in the result array.

from https://www.php.net/manual/en/function.array-merge.php

As replacement we can use array_replace we have an example here https://www.php.net/manual/en/function.array-merge.php#126628

I normally would add a extra test case, but I found it so small to be given an extra test.

The change will preserve the current functionality but add the ability to add another functionality at the same time