Fix `validate()` returned value

[GuillaumeGomez]

If value is None, it would return None and not False.

[Tethik]

Interesting. Did this cause a security issue upstream? I imagine if code exists like the following it could be bypassed.

if captcha.validate() == False:
    ... # abort
else:
   ... # do stuff

[GuillaumeGomez]

It's exactly what happened to me. ^^'

[Tethik]

I see. I'll raise a security advisory in case other users are affected. Thanks for the patch!