Passing a function for FilterDescriptor's operator field returns array values in string format

[zstoyanova9]

Description

When passing a custom function to the operator field of the FilterDescriptor and the field of the filtered data contains values in the form of an array, the returned values by the function are in a lowercase string format.

This issue starts to reproduce as of v1.7.1 of the kendo-data-query package.
Example with v1.7.0 where the function returns the array values in the form of an array, not a string - https://stackblitz.com/edit/angular-eegdc4zq-knrdlw4k

Steps To Reproduce

1. Open the following StackBlitz example - https://stackblitz.com/edit/angular-eegdc4zq-duy4mds5
2. Observe how the returned values by the function are in the form of a string, despite the field in the data containing an array of values.

Actual Behavior

The array values are returned in a lowercase string format.

Expected Behavior

The array values should be returned in the form of an array, not a string.