def add_header(self, key, value):
        """Populates the HTTP header with lines of text.  Sets the
        status code on this response object if the given argument list
        containes a 'Status' header.

        Example:

        >>> resp.add_header("Content-type", "text/plain")
        >>> resp.add_header("Content-Length", "10500")

        :raises ValueError: This happens when the parameters are
                            not correct.

        """
        key = key.strip()
        if key.find(' ') != -1 or key.find(':') != -1:
            raise ValueError('There should be no spaces in header keys')
        value = value.strip()
        if key.lower() == "status":
            self.set_status(str(value))
        else:
            self.headers.update({key: str(value)})