fix for issue #215. The "next" method must have only one return statement. So all the potential generated statements must be mutualy exclusive.

If two return statements are generated in the generated class code, it result in a compilation error "Unreachable code".
added unit test to EncoderDecoderTestGwt which demonstrates 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
fix for issue #212. JsonTypeInfo and JsonSubTypes annotations must not be searched on current class only.

Theses annotations must be searched on the current class, on all parent classes and interfaces.
Merge pull request #216 from bcourtine/master

fix for issue #215. The "next" method must have only one return statement