See comment in DefaultMixedCaseContentTypeHttpHeaderMapper.
* Copy of SI 4.3.9 mapper with contentType case-insensitive test - when
* ServletServerHttpRequest.getHeaders() encounters a content-type with encoding, it is left
* as lower case; when it has no encoding, it becomes Content-Type.
*
* TODO: Remove when we migrate to Spring Integration 5.0.
See comment in
DefaultMixedCaseContentTypeHttpHeaderMapper
.