input: Write a python function to find the longer string between two given strings.
  assert longer_string("apple", "banana") == "banana"
  assert longer_string("hello", "world") == "hello"
  assert longer_string("short", "longer string") == "longer string"
output: def longer_string(str1, str2):
      if len(str1) >= len(str2):
          return str1
      else:
          return str2