Remove redundant String#dup

Since no destructive methods are being called, `dup` isn’t necessary.

lib/mcp/string_utils.rb

@@ -15,7 +15,7 @@ module MCP
     end
 
     def underscore(camel_cased_word)
-      camel_cased_word.dup
+      camel_cased_word
         .gsub(/([A-Z]+)([A-Z][a-z])/, '\1_\2')
         .gsub(/([a-z\d])([A-Z])/, '\1_\2')
         .tr("-", "_")