code: def add(self, key, value):
          
          self._data.setdefault(key.lower(), []).append((key, value))
docs: Adds a (name, value) pair, doesn't overwrite the value if it already
          exists.
  
          >>> headers = HTTPHeaderDict(foo='bar')
          >>> headers.add('Foo', 'baz')
          >>> headers['foo']
          'bar, baz'