* enforce any particular rules with regards to line folding as stated in the class docs.
   */
  private static final CharMatcher LINEAR_WHITE_SPACE = CharMatcher.anyOf(" \t\r\n");

  // TODO(gak): make these public?
  private static final String APPLICATION_TYPE = "application";
  private static final String AUDIO_TYPE = "audio";
  private static final String IMAGE_TYPE = "image";