Fix splat arguments on Rails 7.1

[francescobbo]

OutputBuffer has been changed to not inherit from String (and using the output safety core_ext from activesupport). With that change, the + has been removed.

This change replaces the += operator with concat, which is available on Strings and OutputBuffers alike.

I've added a test that fails without my patch on Rails 7.1.

[bobmaerten]

Could we expect a release soon, now this is fixed?

[minad]

@bobmaerten I will wait a little longer if more Rails 7.1 related issues pop up, see for example #943.

[gstokkink]

@minad I second the request for a release, this would help us a lot. That other bug you're referring to will be fixed in Rails 7.1.2, which is still a ways off.

[ryoung]

Any chance of a release? This fix has been merged a month ago.

[mgrunberg]

@minad would you consider a release now that Rails 7.1.2 got released? At least a rc.

[minad]

Thanks. I noted the interest in an early release. But note that Rails 7.1.1 will still lead to breakage (#943). (EDIT: I see that Rails 7.1.2 has been released yesterday, so we should definitely proceed with this soon.) If Slim is useful for you, please consider supporting the maintenance of the project.