input: Write a class to modify the case of a given string. The class should include methods to convert the string to uppercase, lowercase, and titlecase. If the input string is "Hello World", the output should be the modified versions of the string in uppercase, lowercase, and titlecase respectively.
  
  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()
  
  if __name__ == '__main__':
      test_string = 'Hello World'
      string_modifier = StringCaseModifier(test_string)
      assert string_modifier.to_upper() == 'HELLO WORLD'
      assert string_modifier.to_lower() == 'hello world'
      assert string_modifier.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()