CVE-2019-13146: field_test gem contains injection vulnerability

The vulnerability stems from the field_test method returning arbitrary user-controlled values without validation. Multiple sources explicitly show vulnerable patterns like Page.where("key = '#{landing_page}'") where landing_page comes from field_test. The method's inability to restrict output to predefined variants makes it the clear injection vector. Though exact implementation details aren't shown in diffs, the method's documented behavior and usage examples in advisories directly implicate it.