input: Write a python function to count the frequency of words in a given string.
  assert word_frequency("Hello world, hello programmer") == {'Hello': 1, 'world,': 1, 'hello': 1, 'programmer': 1}
  assert word_frequency("apple banana apple banana orange") == {'apple': 2, 'banana': 2, 'orange': 1}
  assert word_frequency("test this function test this code") == {'test': 2, 'this': 2, 'function': 1, 'code': 1}
output: def word_frequency(s):
      words = s.split()
      freq = {}
      for word in words:
          if word in freq:
              freq[word] += 1
          else:
              freq[word] = 1
      return freq