input: Write a python function to find the most common word in a string using the Counter class from collections module.
  assert most_common_word("The quick brown fox jumps over the lazy dog. The quick brown fox.") == ('The', 2)
  assert most_common_word("apple banana apple apple orange orange") == ('apple', 3)
  assert most_common_word("one two three one two one") == ('one', 3)
output: def most_common_word(s):
      from collections import Counter
      words = s.split()
      counter = Counter(words)
      most_common = counter.most_common(1)[0]
      return most_common