Fix recaptcha submit
I was (once again) hit with rate limiting captcha request, and this time I was able to fix it all the way to the end!
The only bug that's left is that serde_json trys fails parsing content-type-less empty response as JSON and thus the captcha page doesn't go away but re-requests the captcha. But it works, I'm no longer rate limited!
Fixes #535 (closed)