Fix for CBLModel getters of type 'id' (or Swift 'String')

For some reason the prior fix was only implemented for setter methods,
not getters, and the unit tests didn't notice because they only tested
the untyped setter...
Fixes #648

Source/API/CBLModel+Properties.m

@@ -201,7 +201,7 @@ + (BOOL) hasRelation: (NSString*)relation {
 + (IMP) impForGetterOfProperty: (NSString*)property ofClass: (Class)propertyClass {
     id (^impBlock)(CBLModel*) = nil;
 
-    if (propertyClass == Nil) {
+    if (propertyClass == Nil || propertyClass == [NSObject class]) {
         // Untyped
         return [super impForGetterOfProperty: property ofClass: propertyClass];
     } else if (propertyClass == [NSString class]

Unit-Tests/Model_Tests.m

@@ -344,6 +344,7 @@ - (void) test00_SaveModel {
         AssertEqual(model.url, url);
         AssertEq(model.other, model3);
         AssertEqual(model.others, (@[model2, model3]));
+        AssertEqual(model.untypedObject, @"this is an id");
 
         // Save it and make sure the save didn't trigger a reload:
         AssertEqual(db.unsavedModels, @[model]);