Fix middleware request uri race condition

When writing a new Rails system test I ran into an issue where the
browser would hang and not close, eventually failing the test due to
pending requests. There were no clear pending requests and the requests
mentioned were AJAX calls that should have been made several pages ago.

I believe the cause is an AJAX call being cancelled almost immediately
after it was initiated, but I'm not 100% positive.

Code wise, it looks like the value of `env['REQUEST_URI']` that
increments the counter is different from the `env['REQUEST_URI']` that's
decremented in the `ensure` block.

This resolves the issue by storing the `env['REQUEST_URI']` value in a
variable so it will be consistent between counter calls.

lib/capybara/server/middleware.rb

@@ -53,14 +53,16 @@ module Capybara
         if env['PATH_INFO'] == '/__identify__'
           [200, {}, [@app.object_id.to_s]]
         else
-          @counter.increment(env['REQUEST_URI'])
+          request_uri = env['REQUEST_URI']
+          @counter.increment(request_uri)
+
           begin
             @extended_app.call(env)
           rescue *@server_errors => e
             @error ||= e
             raise e
           ensure
-            @counter.decrement(env['REQUEST_URI'])
+            @counter.decrement(request_uri)
           end
         end
       end