input: Write a function to separate words in a string with a given separator.
  assert string_separator("hello world", ",") == "hello,world,"
  assert string_separator("python is awesome", "-") == "python-is-awesome-"
  assert string_separator("test this function", "_") == "test_this_function_"
output: def string_separator(s, sep):
      return sep.join(s.split()) + sep