JIRA: https://jira.spring.io/browse/INT-4096
* Introduce `spring.integration.readOnly.headers` Integration property
* Introduce `IntegrationMessageHeaderAccessor.setReadOnlyHeaders()` and use it from the overridden `IntegrationMessageHeaderAccessor.isReadOnly()`
* Add `DefaultMessageBuilderFactory.setReadOnlyHeaders()` and delegate the value to the new `MessageBuilder.readOnlyHeaders()`
* Modify `spring.integration.properties` for the `contentType` as a `readOnly` header for testing
* Ensure that provided logic works via an appropriate modification to the `ObjectToJsonTransformerParserTests`
Increase receive timeouts in the `OutboundGatewayFunctionTests`
Fix `FileInboundTransactionTests` for slow `WatchService` issue on Linux/OS X
Also redo the `tmp` dir logic to the `TemporaryFolder` `@ClassRule`
Fix unused `import` in the `FileInboundTransactionTests`
Polishing
d1ed66835d