def header(self):
        ''' :class:`HeaderDict` filled with request headers.

            HeaderDict keys are case insensitive str.title()d
        '''
        if self._header is None:
            self._header = HeaderDict()
            for key, value in self.environ.iteritems():
                if key.startswith('HTTP_'):
                    key = key[5:].replace('_','-').title()
                    self._header[key] = value
        return self._header