Support sending CheckID responses with different identifier

[mgorny]

This is handled properly by a fair number of OpenID 2.0 RPs. It allows the provider to support identifier selection even if a different identifier was passed.

You can observe this e.g. with Google accounts. For example, try to login to OpenID 2-capable site (I tested using the openid-ldap.org test script and stackoverflow) using the identifier mgorny.alt.pl. After logging in to your Google account, Google will allow you to choose that account and the site will use it rather than one specified in domain delegation.