INT-3503 equals & hashCode for ToStringFriendlyJN

JIRA: https://jira.spring.io/browse/INT-3503

**Cherry-pick to 4.0.x**

PR Comments and test
INT-3509 Add Convert Node & DOMSource to Document

JIRA: https://jira.spring.io/browse/INT-3509

**Cherry-pick to 3.0.x and 4.0.x**

Polishing

- Javadoc
- Refactor duplicated code
INT-3508: HTTP: Map `MessageHeaders.CONTENT_TYPE`

JIRA: https://jira.spring.io/browse/INT-3508

Before SI 4.0 and moving `MessageHeaders` to the Spring Messaging the `MessageHeaders.CONTENT_TYPE` had a value as `content-type`,
which was mapped to the HTTP Header `Content-Type` well.
Starting with SF 4.0 `MessageHeaders.CONTENT_TYPE` has a value `contentType`. It doesn't allow to map HTTP headers properly.

* Add `contentType` mapping logic to the `DefaultHttpHeaderMapper`, to map to the `Content-Type` HTTP header and vice versa.
* Fix bug in the `DefaultHttpHeaderMapper#toHeaders`: `ObjectUtils.containsElement` -> `containsElementIgnoreCase`.
Some HTTP servers can return `Content-Type` as `Content-type` or even `content-type`, although it is `Content-Type` anyway.
* Add test-case with `<int:object-to-json-transformer/>`, when the `application/json` value from `MessageHeaders.CONTENT_TYPE` is properly
mapped to the HTTP `Content-Type`. Prior to this fix we had to remap `MessageHeaders.CONTENT_TYPE` to the `Content-Type` manually using `<header-enricher>`

Reformat code

Use only `MessageHeaders.CONTENT_TYPE` for mapping to/from SI

Move `MessageHeaders.CONTENT_TYPE` logic to the `fromHeaders`

Performance Improvements

Remove toLowerCase() calls within loop of `shouldMapHeader()`.

Pre-build lower case versions of arrays.
INT-1198: WebSockets: Server Side

JIRA: https://jira.spring.io/browse/INT-1198
INT-1198: WebSocket: Add namespace support

Polishing components according to the namespace support experience
Parser for `<server-container>` and tests
INT-1198: Add parser tests for `<int-websocket:outbound-channel-adapter>`
Introduce `use-broker` on server side
Polishing according PR comments