mod_admin: Fix redirect to admin page when using admin logon form
mod_admin: Fix redirect to admin page when using admin logon form

Using the new admin logon form overwrote the "page" argument always to
redirect to /admin. This patch fixes that.

Fixes #488

erlanger-ru/erlanger-ru.meta,erlanger-ru/erlanger-ru.meta,erlanger-ru/erlanger-ru.meta,erlanger-ru/erlanger-ru.meta,erlanger-ru/erlanger-ru.meta