Prior to this commit, the various `HttpMessageConverter` instances
configured for a given `RestTemplate` instance could all contribute
`MediaType` values to the "Accept:" request header.

This could lead to duplicate media types in that request header,
cluttering for the HTTP request for no reason.

This commit ensures that only distinct values are added to the request.

Issue: SPR-16690
Closes gh-22320
Closes gh-21231