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`. Previously this fix we had to remap `MessageHeaders.CONTENT_TYPE` to the `Content-Type` manually using `<header-enricher>`

**Cherry-pick to 4.0.x**

The second commit is about code reformatting for affected classes
My thought do nothing for 4.0 because the workaround is simple - using `<header-enricher>`.
But from other side it was behavior change since 4.0, because it worked well before. 
So, maybe we really don't have a choice like make this breaking change to the 4.1 only ?

I agree: need to think how to get deal with both headers in a message. Maybe `Content-Type` must have a precedence?..