Add default headers and transform it to CIMultiDict

def _prepare_headers(
            self,
            headers: Optional[LooseHeaders]) -> 'CIMultiDict[str]':
        """ Add default headers and transform it to CIMultiDict
        """
        # Convert headers to MultiDict
        result = CIMultiDict(self._default_headers)
        if headers:
            if not isinstance(headers, (MultiDictProxy, MultiDict)):
                headers = CIMultiDict(headers)
            added_names = set()  # type: Set[str]
            for key, value in headers.items():
                if key in added_names:
                    result.add(key, value)
                else:
                    result[key] = value
                    added_names.add(key)
        return result