Fix  add-to-cart for grouped products which are sold individually

The add-to-cart button for products sold individually on a grouped product page would always lead to the `"Please choose a product to add to your cart"` error.

Eg: http://cl.ly/image/2C3v320t3Y2y

This fixes that, and also restores the `'single_add_to_cart_text'` filter to the add-to-cart button's text.