Add Server header 

Hi! 

The [SPARQL Endpoint status](http://sparqles.okfn.org/) now tracks different server implementations, and that's a good reason to include a Server header, so that we can actually see how our stuff performs in real life. I've added it to RDF::LinkedData too.

I'm in doubt whether the Plack::Response will ever have a Server header preset, but in case it does, I concat any previous.

I wrote it on the plane from Sydney, and didn't pull first, so I hope it's not too painful to pull.