Adjusts 'formatIs' tester for consistency

Besides 'formatIs' all other testers only check whether their type is part of the JSON
Schema type array. This adjusts 'formatIs' to behave the same instead of forcing the
type to only be of type 'string'.

Author: jdwit

packages/core/src/testers/testers.ts

@@ -144,7 +144,7 @@ export const formatIs = (expectedFormat: string): Tester =>
     schema =>
       !isEmpty(schema) &&
       schema.format === expectedFormat &&
-      schema.type === 'string'
+      hasType(schema, 'string')
   );
 
 /**