Support sending CheckID responses with different identifier

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.