input: Write a python function to count the frequency of words in a given string.
  assert word_frequency("Hello world hello world") == {'Hello': 1, 'world': 1, 'hello': 1, 'world': 1}
  assert word_frequency("test the function test function") == {'test': 2, 'the': 1, 'function': 2}
  assert word_frequency("one fish two fish red fish blue fish") == {'one': 1, 'fish': 4, 'two': 1, 'red': 1, 'blue': 1}
output: def word_frequency(s):
      words = s.split()
      frequency = {}
      for word in words:
          frequency[word] = frequency.get(word, 0) + 1
      return frequency