[rubygems/rubygems] Simplify some code

This method always receives an array, and we require `shellwords`
unconditionally at the top of the file, so `shelljoin` will always be
available.

rubygems/rubygems@05c8ac641d

lib/bundler/gem_helper.rb

@@ -215,8 +215,7 @@ def sh_with_input(cmd)
     def sh(cmd, &block)
       out, status = sh_with_status(cmd, &block)
       unless status.success?
-        cmd = cmd.shelljoin if cmd.respond_to?(:shelljoin)
-        raise("Running `#{cmd}` failed with the following output:\n\n#{out}\n")
+        raise("Running `#{cmd.shelljoin}` failed with the following output:\n\n#{out}\n")
       end
       out
     end