INT-2971: AMPQ 'header-mapper' XSD Polishing

* Change expected type from the `HeaderMapper` to the `AmqpHeaderMapper`
* Add 'header-mapper' mutually exclusivity failing tests
* Change AMQP doc to reference `AmqpHeaderMapper` not `HeaderMapper`

JIRA: https://jira.springsource.org/browse/INT-2971

INT-2971 Polishing

Polish schema docs; add info about STANDARD_*_HEADERS.
Merge pull request #776 from artembilan/INT-2971