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
Conflicts:
spring-integration-core/src/main/java/org/springframework/integration/context/IntegrationProperties.java
spring-integration-core/src/main/resources/META-INF/spring.integration.default.properties
Some polishing and documentation
Localize `readOnly.headers` customization only in the `ObjectToJsonTransformerParserTests-context.xml`
Otherwise ti might affect many other tests and the search for the reason of failure would be enough complicated
Doc Polish
dbcebebf38