input: Write a function to replace all occurrences of a character in a string with the provided replacement character. The function should return the modified string. If the character to be replaced is not found in the string, it should return the original string unchanged. Use the following function signature:
  
  def replace_all_occurrences(str, char, replacement):
  
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Python Programming", 'o', 'a')=="Pythan Pragramming"
  assert replace_all_occurrences("No character to replace", 'z', 'X')=="No character to replace"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("aaaa", 'a', 'b')=="bbbb"
output: def replace_all_occurrences(str, char, replacement):
      return str.replace(char, replacement)