Send a Host: header with the purge request

This allows a setup whereby your .varnishrc specifies a hostname or IP
different to the hostname you're purging; the request will be made to
the Varnish server, but the ban will have the correct hostname.

lib/varnisher/purger.rb

@@ -81,7 +81,9 @@ def send
 
       begin
         http = Net::HTTP.new(hostname, port)
-        response = http.request(@request_method.new(@path))
+        request = @request_method.new(@path)
+        request.add_field("Host", @host)
+        response = http.request(request)
       rescue Timeout::Error
         return false
       end