Added support of simple AX values.

I added the support of simple values for AX:

The pre-existing behavior is the user set a list (with `setValues` or `addValue`) the AX response will be sent with:
"openid.ax.count.<alias>" and "openid.ax.value.<alias>"

This pull request add the support of single value, with `setValue`. The AX response will only contain:
"openid.ax.value.<alias>"