“Want to do this for a different person?” displays new member signup form even when selected contact is a current member

This applies to a membership contribution page. The “Want to do this for a different person” feature does not check the selected contact’s membership status to determine whether to show the signup or renewal form; it always displays the new member signup form.
Steps to reproduce:
1. Set up a contribution page for member signups and renewals with different titles and introductory messages for new memberships and renewals
2. Create a contact ‘Contact1’ and add a membership to Contact1
3. Create a contact ‘Contact2’ and add a membership to Contact2
4. Set privileges for Contact1 sufficiently to be able to renew Contact2’s membership
5. Go to an online membership renewal contribution page, logged in as Contact1—membership renewal form should be displayed
6. Click on ‘Not Contact1, or want to do this for a different person?’ link
7. Select Contact2
8. A new member signup form will now be displayed (i.e. the title and intro message will be those applicable to new memberships), with Contact2’s details. However this should be a renewal form as Contact2 is an existing member.

Tested on http://d46.demo.civicrm.org (4.6.10) and locally on 4.6.10 for Joomla.