Send frame as request paramater as well

Here's what happens: When a client sends simple command without any
values in XML, registry can drop the `raw_frame` parameter
completely. If so, it relies on `frame` parameter being passed on to
create XML document.

https://github.com/internetee/registry/blame/ad823391b75509d5be20ee6ef217aa4f35a4c994/lib/epp_constraint.rb#L14

Fix involves sending the XML string twice, as `frame` and
`raw_frame`, the same as `mod_epp` did.

apps/epp_proxy/priv/test_backend_app/epp_server.rb

@@ -16,7 +16,10 @@ class EppServer < Roda
       end
 
       r.post "logout" do
-        render("session/logout")
+        # Additional check if passes empty frame
+        if r.params['frame']
+          render("session/logout")
+        end
       end
     end