updated test case

YYModel/NSObject+YYModel.m

@@ -1198,10 +1198,8 @@ static id ModelToJSONObjectRecursive(NSObject *model) {
                 subDic = superDic[key];
                 if (subDic) {
                     if ([subDic isKindOfClass:[NSDictionary class]]) {
-                        if (![subDic isKindOfClass:[NSMutableDictionary class]]) {
-                            subDic = subDic.mutableCopy;
-                            superDic[key] = subDic;
-                        }
+                        subDic = subDic.mutableCopy;
+                        superDic[key] = subDic;
                     } else {
                         break;
                     }

YYModelTests/YYTestModelToJSON.m

@@ -138,6 +138,9 @@
     XCTAssert([ext[@"b"] isEqualToString:@"b"]);
     XCTAssert([ext[@"a"] isEqualToString:@"a"] || [ext[@"a"] isEqualToString:@"c"]);
     
+    model.f = @{@"g" : @""};
+    dic = [model yy_modelToJSONObject];
+    
 }
 
 - (void)testDate {