use restclient , a plugin for fire fox and add the http headers as Accept:application/json ,content-type: application/json.
You have to find a way to tell firefox to set the content-type to application/json. The error indicates that it's sending something else.
Sorry for the late answer, but it may serve to others.
You should doublecheck than your Customer class is annotated with JAXB's @XmlRootElement, since jackson needs it to deserialize JSON message.