identity_url should be used for uid instead of display_identifier

[ntippie]

https://github.com/omniauth/omniauth-openid/blob/8a46fa98dabe3ea7ced336fef1201ab4870adb15/lib/omniauth/strategies/open_id.rb#L69

As noted in the OpenID spec, the claimed identifier should be used (#identity_url instead of #display_identifier).