Fixed the handling of flash messages

[javiereguiluz]

Fixes the #859 merge.

[xabbuh]

I do not understand this change. I mean the flash entries will probably be created when processing a POST request, but the reason we register them is that we redirect at the end and we need to display them in response for the subsequent event which will be a GET request then. So with this change the flash messages are likely to never be shown.

[sebastianblum]

Hello @javiereguiluz and @xabbuh

This fix does only work with 307 redirects, see https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/307

Submit form: POST /url - store flash message and redirect with 307 to /thank-you
307 redirect: POST /thank-you - shows the flash message

With an 302 redirect, the redirect will switch to an GET-Request and so the flash message will be hidden.
Submit form: POST /url - store flash message and redirect with 302 to /thank-you
302 redirect: GET /thank-you - doesn't show the flash message

But with the 307 request, the post body will be resend. That looks wrong to me.

[stof]

Posting to a thank-you page looks wrong to me btw (that page should probably not even accept POST requests)