added unit test to EncoderDecoderTestGwt which demonstrates a problem wi...

Added unit test "testJsonSubTypesWithInterfaceUsingConcreteImplementationCodec" to EncoderDecoderTestGwt which fails at the moment because of a problem with missing @class attribute in json when using an encoder of a concrete implementation instead of a super class. This leads to exceptions when decoding the created json to java object with jackson 2.3.2 on server side in a spring mvc controller.

In fact this is a problem if you have a RestService for the concrete subclass and a corresponding spring mvc controller on server side.

According to implementation of jackson 2.3.x the @class attribute should be added ever, when any of the super classes has a @JsonTypeInfo and @JsonSubTypes annotation.

Tried to fix it myself yesterday, but i am not familiar enough with GWT Generators and the resty gwt codebase to figure out were to put this in.