code: def _prepare_headers(
              self,
              headers: Optional[LooseHeaders]) -> 'CIMultiDict[str]':
          
          # 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
docs: Add default headers and transform it to CIMultiDict