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
INT-2455: HTTP outbound 'encode-uri' flag support

Now `<http:outbound-channel-adapter>` and `<http:outbound-gateway>` provide
`encode-uri="false"` option do not encode request uri.
It can be useful in some scenarios with non standard URL, e.g. RabbitMQ REST API:
'http://foo.RabbitMQ.com/api/queues/%2f/bar.queue'

https://jira.springsource.org/browse/INT-2455
INT-2455: Polishing according PR comments
INT-2455: Add 'encode-uri' to the Manual
INT-2455: 'encode-uri' in the Reference Manual