update req.backend to contain .host

[david raistrick]

re #varnish discussion about req.backend (when being set in a http header) 
only contains the director (if it's a director) and doesn't allow any way 
to see the actual backend.

<Mithrandir> keen: maybe the string representation of the various backends
              should include the .host field.



thanks. :)

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david raistrick        http://www.netmeister.org/news/learn2quote.html
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