Honor If-Modified-Since in request headers

[hyunilmmt]

This would be very useful in combination with Last-modified in response headers, especially for SEO. The challenge would be how to elegantly decide whether the resource has been modified or not.

[mjpieters]

I'm not sure that this project can accomodate this without additional application-specific work. I strongly suspect that a custom key builder (which has access to the request) could do this, but that's beyond the scope of the project.

I'll be reworking the documentation, perhaps someone could contribute an example on how you could do this with a keybuilder?