def add_conditional_headers(req, opts)

      # if path is cached send since and/or match

      e = @cache[opts[:c_uri]]

      return unless e # not cached

      req['If-Modified-Since'] = e.lastmod if e.lastmod
      req['If-None-Match'] = e.etag if e.etag

      opts[:c_cached] = e
    end
def add_conditional_headers(req, opts)

      # if path is cached send since and/or match

      e = @cache[opts[:c_uri]]

      return unless e # not cached

      req['If-Modified-Since'] = e.lastmod if e.lastmod
      req['If-None-Match'] = e.etag if e.etag

      opts[:c_cached] = e
    end
If the representation has already been gotten, send
 potential If-Modified-Since and/or If-None-Match.