input: Write a python function to find the most common word in a given sentence and its frequency.
  assert frequency("the quick brown fox jumps over the lazy dog") == ("the", 2)
  assert frequency("hello world hello") == ("hello", 2)
  assert frequency("test test test case case") == ("test", 3)
output: def frequency(sentence):
      from collections import Counter
      words = sentence.split()
      count = Counter(words)
      most_common_word, freq = count.most_common(1)[0]
      return most_common_word, freq