You can try with WildFly 8 alpha2 which has RESTEasy 3.0.1 already bundled.
Oterwise did you folow docs about upgrading RestEasy in AS7 http://docs.jboss.org/resteasy/docs/3.0.1.Final/userguide/html_single/index.html#upgrading-as7
there are modules provided in distribution that you should put in your AS7 modules directory...
Ok, I'll try WidlFly.
Anyway, I upgraded RestEasy follwing that docs twice. The first one with my old AS7 install and the second with a fresh installation and no one worked fine until I put Jackson2 libraries into my WEB-INF/lib folder.
Thanks for you suggestion.
I've try wildfly 8 alpha 2 and the issue persist.
The only way I have discovered to fix it is with the solution that I suggest in the initial message, you must to include all Jackson2 libraries in WEB-INF/lib and exclude org.jboss.resteasy.resteasy-jackson-provider.
I will continue investigating...