INT-2455: Support HTTP Outbound 'encode-uri' Flag

The `<http:outbound-channel-adapter>` and `<http:outbound-gateway>` now provide
an `encode-uri` option that controls whether the request uri is encoded
(default `true`).

It can be useful in some scenarios with a non standard URL, e.g. RabbitMQ REST API:
'http://foo.RabbitMQ.com/api/queues/%2f/bar.queue' where we must not
encode the '%'.

https://jira.springsource.org/browse/INT-2455

INT-2455: Polishing based on PR comments

INT-2455: 'encode-uri' in the Reference Manual
Merge pull request #755 from artembilan/INT-2455

* INT-2455:
  INT-2455: Support HTTP Outbound 'encode-uri' Flag