DynamicMBean attributes should convert to Java classes correctly

[dgolombek]

I get #<NoMethodError: undefined method `new' for "java.lang.Integer":String> when trying to use DynamicMBean.[rw]_attribute, since to_java_type returns a String, but it's being executed as if it returns a Class. I'll try to get a patch submitted soon, but will probably be ~2 weeks due to other commitments. Alternatively, we could update #7 and use that, which I think is a cleaner fix anyways.