input: Write a python function to replace spaces in a string with a specified character.
  assert replace_spaces("This is a test string", "X") == "ThisXisXaXtestXstring"
  assert replace_spaces("Hello World", "-") == "Hello-World"
  assert replace_spaces("Spaces  here   and there", "_") == "Spaces__here___and_there"
output: def replace_spaces(s, char):
      return s.replace(" ", char)