Fix for saving of Instance.extra fields.

When an extra value in a to-many relationship table has been changed, the saving logic breaks in the instance class. This fix worked for me. In the snippet I changed, opts.extra_info.id is an array containing the column names of each of the extra fields that need to be saved. I changed it so that instead of comparing the iterator variable i to the id array (which didn't make sense), you instead compare to the length of the array.