Enhance OStringSerializerHelperTest by new tests

There are lots of possibilities of enhancements of OStringSerializerHelper. But it's widely used and first of all we need to cover it by tests.

New test currently is passing. But, please concider following my comment: 

```
Following tests will be nice to support, but currently it's not supported!!!
{param1 :value1, param2 :value2}
{param1 : value1, param2 : value2}
{param1 : "value1", param2 : "value2"}
{"param1" : "value1", "param2" : "value2"}
{param1 : "value1\\value1", param2 : "value2\\value2"}
```