input: Write a function to create a Python class that can modify the case of a given string. The class should have methods to convert the string to uppercase, lowercase, and title case.
  
  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()
  
  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("this is a test").to_title() == "This Is A Test"
  assert StringCaseModifier("").to_upper() == ""
  assert StringCaseModifier("").to_lower() == ""
  assert StringCaseModifier("").to_title() == ""
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()