Ensure registration of Jackson modules in the environment of an embedded Tomcat

Separate the configuration/creation of the default ObjectMapper bean from the registration of Jackson modules. 
This is to avoid circular creations of the default ObjectMapper bean (and thus failing to obtain the ObjectMapper and registering the module(s)).

Sorry for not writing any tests to verify this, but I'm still new to Spring Boot and don't know yet how to write such a test with the embedded Tomcat.