Membership Batch Data Entry: Renewal not working

Using the Batch Data Entry for memberships doesn't renew correctly memberships.

Steps to reproduce:

1. create a new membership for a contact with a join_date before the last membership period; for example:
   • join_date: 2010-01-01
   • start_date: 2015-01-01
   • end_date: 2015-12-31
2. create a new batch data entry for memberships
3. fill the form for your contact's renewal and leave start_date and end_date empty
4. complete the batch

Expected results: your contact's membership should be renewed for the current year.

Observed results:

• start_date: 2010-01-01 (which is the same as join_date)
• end_date: 2010-12-31 (which is start_date + 1 year).