CRM-20395 Contact 'World Region' Field not functioning properly in Search Builder

To reproduce:

1. 'Search' -> 'Search Builder'.
2. Search for 'Contacts' -> 'World Region' -> '='.

Upon doing the above, the user is presented with a text box instead of the expected dropdown <select> menu.

If the user proceeds to use the unexpected textbox and manually type in the World Region they are searching for, they get an error of the following kind:

Europe and Central Asia is not of the type Positive

This has been reproduced successfully on the test site at http://wpmaster.demo.civicrm.org