Add fastpath for in place scrub

Author: Lillian Zhang

src/main/ruby/truffleruby/core/string.rb

@@ -1019,6 +1019,7 @@ def scrub(replace = nil, &block)
   end
 
   def scrub!(replace = nil, &block)
+    return self if valid_encoding?
     replace(scrub(replace, &block))
     self
   end