I couldn't tell you for certain without knowing how the JSON deserialization is working? the field's name is "customer-id", but the property is named "customerId"...is your object mapper configured to convert the dashed field name to a camel-cased property name?
In short, I believe your problem has less to do with MockRestServiceServer and more to do with how you've implementing the Jackson mapping code. (I'm assuming that you're using Jackson for JSON processing.)
Spring Social Project Lead