RPDE feeds do not set Cache-Control headers correctly #137

Per the recommendations for scaling an RPDE feed, the Cache-Control header for all pages in an RPDE feed should have a value of public, max-age=3600, other than the last page, which should have a value of public, max-age=8

However, the ResponseContent returned from the ResponseContent.RpdeResponse method does not contain any way of indicating whether the page being returned is the last page. It's therefore not possible to vary the cache settings correctly.

Suggest that ResponseContent should have an additional Headers property on it, probably of type Dictionary<string, string> which can then be filled with the correct Cache-Control headers by GetOpenDataRPDEPageForFeed depending on whether this is the last page or not (ie, if the call to RouteOpenDataRPDEPageForFeed returns an empty or non-empty page). The GetContentResult helpers can then add the required headers where necessary.