microptimize options and default merge and other string key array merges

This unifies the way string-key-based options are merged across the core.
Those should be merged using `+` as already done everywhere else.
Additionally is 4-5x faster than array_merge, as well.

`+` operator can be used with options where
* options are a flat array of string based keys
* the resulting order doesn't matter

So I ignored some occurrences in PaginatorHelper - as well as in some other places - to not have to touch/modify any test cases and to keep it 100% BC.