input: Write a python function to find the most frequent word in a given string.
  assert most_frequent_word("Code Generation is an important task in the AI research")==['Generation']
  assert most_frequent_word("Hello world hello")==['Hello', 'hello']
  assert most_frequent_word("Test test TEST")==['Test', 'test', 'TEST']
output: def most_frequent_word(s):
      from collections import Counter
      words = s.split()
      counts = Counter(words)
      max_count = max(counts.values())
      return [word for word, count in counts.items() if count == max_count]