INT-1362 HTTP Inbound: Additional EvalCtx Vars.

JIRA: https://jira.springsource.org/browse/INT-1362, https://jira.springsource.org/browse/INT-2469

* Add to the `EvaluationContext` of HTTP Inbound Endpoints additional request variables
* Tests and documentation

INT-2669: Mention SpEL variables in JavaDocs

JIRA: https://jira.springsource.org/browse/INT-2669

INT-1362 Doc Polishing