Add Sftp Server Tests

Using Apache Mina
SftpServerTests: BASE64Decoder -> apache..Base64
INT-2634: Test for gateway's `Map` parameter

JIRA: https://jira.spring.io/browse/INT-2634

The Map entries with non-String keys are now skipped just with WARN message,
instead of `IllegalArgumentException` as it was before
INT-3200: SFTP: use `byte[]` for the PrivateKey

JIRA: https://jira.spring.io/browse/INT-3200

INT-3200: Remove reduntant test case

INT-3200: remove trailing space
Update CONTRIBUTING.md
INT-1197: Add support for WebSockets: client side

JIRA: https://jira.spring.io/browse/INT-1197
INT-1197: Add JavaDocs and some polishing
Further changes

* Upgrade to SF 4.1
* Rename `SubProtocolHandlerContainer` to the `SubProtocolHandlerRegistry`
* Add `MessageConverter` support to the adapters
* Add `ClientWebSocketContainer.openConnectionException` to be thrown on `getSession` request
* Add `ClientWebSocketContainer.connectionLatch` to wait the connection establishing on first request.
Since the WebSocket connection process is run in the separate Thread, we need to wait it from the first `message send` do not lose the message, if connection hasn't been established yet.
INT-1197: Add `PassThruSubProtocolHandler`

Add `StompIntegrationTests`
INT-1197: Polishing according PR comments
INT-1197: Use SockJs from tests

Fix other detected vulnerabilities