Delegate valid_encoding? for ActionView::OutputBuffer

[enriikke]

Summary

Adds ActionView::OutputBuffer#valid_encoding? as a delegated method.

ActionView::OutputBuffer used to inherit from ActiveSupport::SafeBuffer, which itself inherits from String, so it used to expose all public String methods. This was refactored as part of #45614, however I don't think that leaving out some string methods was intentional (it already delegates some of them).

We have encoding validation checks at GitHub so I think exposing what the encoding is (encoding) and whether it's valid (valid_encoding?) would be reasonable and something we need. 😃

[enriikke]

Working through this a bit more and I think we could find a different way to approach it without having to add another delegated method here, specially since this is meant to be an internal class anyway! 😄