INT-3461: Add `rometools` Dependency for SF 4.1

JIRA: https://jira.spring.io/browse/INT-3461

To test it: `gradlew :spring-integration-http:testAll -PspringVersion=4.1.0.BUILD-SNAPSHOT`

INT-3461: Add CP condition for `rometools` into the `HttpRequestHandlingEndpointSupport`

INT-3461: Add CLASSPATH check for `RequestEntity` to determine the SF 4.1
INT-3417: Polishing Docs for `MessageHeaders`

JIRA: https://jira.spring.io/browse/INT-3417

Polishing

Add note about using the accessor.
INT-3448: Add `@Async` test for `@Gateway`

JIRA: https://jira.spring.io/browse/INT-3448

INT-3448: Addressing PR comments

Late Resolution of @Publisher.defaultChannel

Early resolution of the publisher default channel caused
`@ Configuration` factory beans to be instantiated before
Spring Integration bean post processors had been registered.

Use the channel name instead of channel reference and resolve
it when processing the first publish.

Merge branch 'INT-3448' of https://github.com/garyrussell/spring-integration into INT-3448

Conflicts:
	spring-integration-core/src/test/java/org/springframework/integration/configuration/EnableIntegrationTests.java

INT-3448: Polishing