docs don't mention the need to add amqp_ prefix to headers in order to have DefaultAmqpHeaderMapper add it to the message

I was attempting to have a header enricher add a content type header to a message that was subsequently being handled by an outbound AMQP channel adapter and found that various combinations of contentType and content-type for the header name weren't working.

After digging through DefaultAmqpHeaderMapper and AmqpHeaders I realized that it needs an 'amqp_' prefix in order for the header mapper to automatically copy it. It would be good for this to be mentioned in the docs, or a better way of handling standard AMQP headers to be developed.

Here is a snippet illustrating the final config that worked:

<int:chain input-channel="input" output-channel="output">
    <int:object-to-json-transformer />
    <int:header-enricher>
        <int:header name="amqp_contentType" value="application/json" overwrite="true" />
    </int:header-enricher>
</int:chain>