State parameter incorrectly missing for some backends

[jlafon]

It looks like some of the backends are configured not to make use of the state parameter, even though the corresponding authorization servers support it. I didn't check every single one, but here are two examples:

• The backend for angle.co does not include it, but the state parameter is present in their documentation.
• Similarly, the backend for spotify.com and the corresponding documentation.

[omab]

I've enabled the state parameters in the mentioned backend. It's hard to track all the APIs changes for a single dev :-/

[jlafon]

Thanks!