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>`

The second commit is about code reformatting for affected classes
I did a little profiling with YourKit and these changes (https://github.com/garyrussell/spring-integration/commit/547b142bfeb8005cb48a468475945c7dd2df20ea) reduced the test run time from 4.3 seconds to 0.4 seconds.

Interestingly, by far, the biggest gain was moving `toLowerCase()` calls out of the loop in `shouldMapHeader()`.