Foundation: repair Darwin compatibility test suite

Explicitly convert the field to `Bool` from `DarwinBoolean` for encoding
to ensure serialisability.

Author: compnerd

Foundation/DateIntervalFormatter.swift

@@ -164,10 +164,10 @@ open class DateIntervalFormatter: Formatter {
 
         let dateTemplateFromStylesNS = dateTemplateFromStyles?.takeRetainedValue()._nsObject
         aCoder.encode(dateTemplateFromStylesNS, forKey: "NS.dateTemplateFromStyles")
-        
-        aCoder.encode(modified, forKey: "NS.modified");
-        aCoder.encode(useTemplate, forKey: "NS.useTemplate")
-        
+
+        aCoder.encode(modified == true, forKey: "NS.modified");
+        aCoder.encode(useTemplate == true, forKey: "NS.useTemplate")
+
         let localeNS = locale?.takeRetainedValue()._nsObject
         aCoder.encode(localeNS, forKey: "NS.locale")