Use `FrozenError` instead of `RuntimeError` in `String#rstrip!`
MODIFY
mrbgems/mruby-string-ext/mrblib/string.rb
mrbgems/mruby-string-ext/mrblib/string.rb
@@ -93,7 +93,7 @@ class String
# "hello".rstrip! #=> nil
#
def rstrip!
- raise RuntimeError, "can't modify frozen String" if frozen?
+ raise FrozenError, "can't modify frozen String" if frozen?
s = self.rstrip
(s == self) ? nil : self.replace(s)
end

Use `FrozenError` instead of `RuntimeError` in `String#rstrip!`