avoid ivars and extra array for response

This adds is needless overhead and prevents thread-safety when
used with a thread-safe Aggregate-like object is passed via
:aggregate on initialize.

lib/rack/aggregate/context.rb

@@ -31,10 +31,10 @@ def call(env)
         end
 
         start = Time.now
-        @status, @headers, @response = @app.call(env)
+        response = @app.call(env)
         @aggregate << (Time.now - start) * 1000
 
-        [@status, @headers, @response]
+        response
       end
     end
   end