Fix KeyedService.AnyKey support

[sunghwan2789]

DecorationException is thrown if a non-keyed service and a keyed service are registered.

I think this is a bug since 5.0.3

This PR fix Decorate not to try decorate keyed service

[khellang]

Hmm, I keep staring at the code, but I'm having trouble groking what's actually changed here. Why is the string equality check not enough?

I see the cast probably should be pulled out as a pattern cast and only run the string equality check if the key is actually a string, but I'm having trouble seeing why the reference equality check is necessary. Is it an optimization? To catch nulls?

[khellang]

Ah, after looking at the test (thanks for including it ❤️) I get it. There's a KeyedService.AnyKey sentinel object that's being checked for. I wasn't aware of this value.

Can we maybe include a comment to point out why those checks are there? 😄

[sunghwan2789]

@khellang While adding a comment, I found that object.Equals is a better choice as it checks for ordinal string equality and can pass the test case as well.