It turns out that `contentType` header (as well as `Accept`) may be
converted into `MimeType` upstream before the message is sent into HTTP
Outbound Channel Adapter meanwhile the logic is only to support String
and MediaType
* Change the `DefaultHttpHeaderMapper` to support more generic `MimeType`
for the `Content-Type` and `Accept` HTTP headers which can simply be
converted to the expected `MediaType`
**Cherry-pick to 5.1.x & 5.0.x**
a75f0808fd