dev/core#1718 membership batch entry join date fix

[lcdservices]

Overview

When using the membership batch entry tool, if you select a contact with an existing membership it prepopulates with existing data. The join date is not prefilled because of an incorrect field name reference.

Before

Join date is not populated with existing data.

After

Join date is populated with existing data.

[civibot]

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[pradpnayak]

Did a ui test and it seems to populate member since with correct date. Good to merge!

Before:

After:

[yashodha]

@lcdservices @pradpnayak looks good, merging this now. Thanks!