Null values are deserialized as the literal string "null"

[ghost]

Scenario:

1. On the producer side, a null value is associated with the key "foo" and sent -> message.put("foo", null);
2. On the consumer side, when the value is read out of the injected message -> String x = message.get("foo") -> we get the literal string "null"

This was found using the master branch of forklift.