CatalogRule: Saving a rule with multiple options on a multi-select will only select the first option

[koenner01]

If you create a catalog price rule, add a multi-select as condition and you choose multiple options as the value then only the first option will be kept as the value when saving the rule.

Preconditions

1. MG2 CE 2.1.7 (also tested on 2.1.5, 2.1.6)
2. PHP7.0

Steps to reproduce

1. Create a multi-select attribute with at least 2 options and set it to be used for promo
2. Create a catalog price rule and add a condition containing our multi-select option
3. Choose an operator (doesn't matter which, I tested with 'contains' and 'is one of')
4. Select at least 2 options as value
5. Save (and continue) the rule

Expected result

1. The saved rule's conditions should contain all our selected options for the multi-select

Actual result

1. Only the first option has been saved on the rule

[koenner01]

duplicate #8740

[koenner01]

duplicate #7723