String from `wsgi.input`.

def body(self):
        """ String from `wsgi.input`.
        """
        if self._body is None:
            if self._fieldstorage is not None:
                raise ReadBodyTwiceError()

            clength = int(self.environ('CONTENT_LENGTH') or 0)
            self._body = self._environ['wsgi.input'].read(clength)
            if isinstance(self._body, bytes):
                self._body = self._body.decode('utf8')

        return self._body