avoid duplicate requests to the cache proxy. 

fix #125
ah, indeed the headers are key-value and not "Key: Value" as with curl
command line. i will fix that. not sure if changing the case of headers
is a good idea however. as varnish regex are case sensitive, this could
actually matter. people should pay attention. the worst case are
duplicate requests so i would rather leave that out.

> case insensitive url?

according to https://www.varnish-cache.org/docs/2.1/faq/general.html
"Why are regular expressions case-sensitive?" varnish does treat urls as
case sensitive. i dont want to treat them insensitive - in a sane
application it won't matter be different case for the same request.

> case insensitive method?

the method is really an edge case too, as the requests are created
through the AbstractProxyClient methods. so the only case where this
would matter is when somebody extends the client and adds a method that
allows to pass in the http method, or adds two methods for the same http
method with different case. i think both are really really edge cases
(and would in the worst case lead to a double request to varnish).