Backport of ContentDisposition fixes to HttpHeaders #24580
Description
ContentDisposition was created in 5.0, supporting all Content-Disposition types including "form-data", "inline" and "attachment" for use in client and server scenarios. Prior to that, HttpHeaders did have support for Content-Disposition of type "form-data", for use in client scenarios with the RestTemplate. This ticket is to review fixes in ContentDisposition that may be worth backporting to HttpHeaders in the 4.3.x branch for consistency.