Bindings for header exchanges which test for the *presence* of a header are not mocked correctly

[hjohn]

Please see this PR: #256

For example for a RabbitListener defined like this:

                @QueueBinding(
                        arguments = {
                                @Argument(name = "my-header") // check for presence of the header, value is irrelevant
                        },
                        exchange = @Exchange(name = "my-exchange", type = ExchangeTypes.HEADERS),
                        value = @Queue(value = "my-queue")
                )

Note that @Argument does not specify a value.