Fixed id reuse (causing object overwrite) issue for inner objects defined in <constructor-arg> without an explicit id value.

I was using spring-python to create objects based on XML files, and came across errors when defining inner objects in <constructor-arg> without specifying id values. It seems the inner objects without explicit id values could have the same id values generated, e.g. someprefix.constr.\<anonymous\>, thus actually use the same object. To make such inner object id optional, I made the code changes to generate unique ids.