Sets a header on the request and returns the request itself.
        The header key will be canonicalized before use. (see also: canonicalize_header)

        Keyword arguments:
        key -- the header's name
        value -- the string value for the header

def with_header(self, key, value):
        """Sets a header on the request and returns the request itself.
        The header key will be canonicalized before use. (see also: canonicalize_header)

        Keyword arguments:
        key -- the header's name
        value -- the string value for the header
        """
        self.header[canonicalize_header(key)] = value
        return self