util: format to not inspect strings if first argument is non-string

current:
  util.format('a', 'b'); // "a b"
  util.format({}, 'a', 'b'); // "{} 'a' 'b'"

changed to:
  util.format('a', 'b'); // "a b"
  util.format({}, 'a', 'b'); // "{} a b"