Make String#rstrip{,!} raise Encoding::CompatibilityError for broken coderange

[jeremyevans]

It's questionable whether we want to allow rstrip to work for strings where the broken coderange occurs before the trailing whitespace and not after, but this approach is probably simpler, and I don't think users should expect string operations like rstrip to work on broken strings.

In some cases, this changes rstrip to raise Encoding::CompatibilityError instead of ArgumentError. However, as the problem is related to an encoding issue in the receiver, and due not due to an issue with an argument, I think Encoding::CompatibilityError is the more appropriate error.

Fixes [Bug #18931]