Add support for per form csrf tokens

[jkowens]

Resolves #1616. Makes CSRF tokens more secure (see: rails/rails#22275).

To generate a per form authenticity token, pass the path and optionally the form method (method defaults to post):

Rack::Protection::AuthenticityToken.token(session, path: '/foo')

[jkowens]

@namusyaka any chance we could release this soon if it looks good? I found this will be really useful for a Rails project where I depend on omniauth (which will be using rack-protection in the 2.0 release).

See: omniauth/omniauth#1017

[jkowens]

@namusyaka thanks for the review. I made updates based on your comments 👍

[namusyaka]

Sorry I'll take a look at this next month.. 🙇

[namusyaka]

Approving for the code.

@jkowens I'm sorry for the late reply.
@rkh Could you also take a look at this?

[rkh]

Code looks good to me.

[mobilutz]

Thanks @jkowens for working on this.
And thanks @namusyaka for merging this 😉

Looking forward to the new rack-protection version ...

[mobilutz]

Does anyone know, if there will be a release before v3.0.0 with this change?

I know that v3.0.0 is being worked on, but maybe we can have a 2.2.0 release in the meantime?

Thanks