Better handling of custom vnd media types

The current implementation of `getShortMimeType` generates names that are not valid as Java identifier so it's basically impossible to use custom vnd. media types with the generator.

I've added unit tests as well, first to capture the current behaviour, proves it doesn't change then test the new behaviour.

This PR builds on https://github.com/mulesoft/raml-jaxrs-codegen/pull/32 which is supposed to be pulled.