[Openid-specs-fapi] Issue #88: Sender constraining the code (openid/fapi)

[Nat Sakimura]

New issue 88: Sender constraining the code
https://bitbucket.org/openid/fapi/issues/88/sender-constraining-the-code

Nat Sakimura:

For AS that provides request object registration endpoint, the AS can actually bind the `code` to the client certificate that was used to authenticate at the request object registration endpoint. This mitigates the `code` phishing attack.