Allow bypassing content negotiation process

[petermorlion]

This PR allows to set a MediaType property like this:

[CacheOutput(ClientTimeSpan = 50, ServerTimeSpan = 50, MediaType = "image/jpeg")]
public HttpResponseMessage Get(int id)
{
    var response = new HttpResponseMessage(HttpStatusCode.OK);
    response.Content = GetImage(id); // e.g. StreamContent, ByteArrayContent,...
    response.Content.Headers.ContentType = new MediaTypeHeaderValue("image/jpeg");
    return response;
}

This wil store the item as the given media type and always return it in this way.

[filipw]

perfect - thanks!