Link render element overwrites CSS class

Here's a patch created on D10.3, but I suspect it applies fine to 11.x

Changes are made on on 11.x (our main development branch) first, and are then back ported as needed according to our policies.

Argh. Apparently core (and maybe contrib) creates attribute array class elements as both strings and arrays, so a cast to (array) is needed. I'd call that a bug, but that's another issue.

Here's a revised patch.

thank you for reporting

Fixes should be in MRs vs patches

Also will need a test case to show the problem.

I've just created a MR and added a Kernel test :)