Fix issue with persistable reference handling in converters;

This merge request is link with the another one in os-core-common. The issue was found in the Storage GA and concerned feature of the Gson lib, namely getAsString() method of the com.google.gson.JsonElement class with the subclasses (JsonPrimitive, JsonObject, etc). The thing is that the method overriden safely only for JsomPrimitive subclass, and if the field mapped as JsonObject one (persistableReference in the 'meta' part, in our case) this call throws UnsupportedOperationException. My enhancement prevents undesirable throwing the exception. Also tests were extended