input: Write a Python class to modify the case of the string. The class should have methods to convert the string to uppercase, lowercase, and title case.
  if __name__ == '__main__':
      test_string = 'Hello World'
      string_modifier = StringCaseModifier(test_string)
      print(string_modifier.to_upper())
      print(string_modifier.to_lower())
      print(string_modifier.to_title())
  assert StringCaseModifier('Hello World').to_upper() == 'HELLO WORLD'
  assert StringCaseModifier('Hello World').to_lower() == 'hello world'
  assert StringCaseModifier('Hello World').to_title() == 'Hello World'
  assert StringCaseModifier('hello world').to_upper() == 'HELLO WORLD'
  assert StringCaseModifier('HELLO WORLD').to_lower() == 'hello world'
  assert StringCaseModifier('hello world').to_title() == 'Hello World'
output: class StringCaseModifier:
      def __init__(self, string):
          self.string = string
  
      def to_upper(self):
          return self.string.upper()
  
      def to_lower(self):
          return self.string.lower()
  
      def to_title(self):
          return self.string.title()