input: Write a python function to merge two strings into a set of unique characters and return as a single string.
  assert merge_strings("abc", "def") == 'abcdef'
  assert merge_strings("hello", "world") == 'hewlord'
  assert merge_strings("python", "java") == 'pythonavj'
output: def merge_strings(s1, s2):
      return ''.join(sorted(set(s1 + s2)))