code: def add(self, key, val):
          
          key_lower = key.lower()
          new_vals = [key, val]
          # Keep the common case aka no item present as fast as possible
          vals = self._container.setdefault(key_lower, new_vals)
          if new_vals is not vals:
              vals.append(val)
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'