Add to Cart form select list double-escapes HTML entities

The array of '#options' form a Drupal select element gets passed though check_plain() inside the form_select_options() function in includes/form.inc. So the option values should not be passed through check_plain() when defining the select form element.

The code at line 1648 to 1650 in commerce_cart.module should be changed from:

        foreach ($products as $product_id => $product) {
          $options[$product_id] = check_plain($product->title);
        }

to

        foreach ($products as $product_id => $product) {
          $options[$product_id] = $product->title;
        }

and the code from 1676 to 1678 should similarly read as follows, without the check_plain():

        foreach ($products as $product_id => $product) {
          $options[$product_id] = $product->title;
        }

They're similar in that they're both double-quoting bugs, but they're quite different places in the code.

That #1089328: HTML entities aren't handled correctly when adding a product reference is for the Product Reference drop-down selector, a field selector.
This #1365202: Add to Cart form select list double-escapes HTML entities is for the Add to Cart drop-down selector on a product display page.

The patch above is all that's needed to fix this problem. Items in a form '#options' array should NOT be passed through check_plain() as the form code does that later.