MultiPartRepresentation.setBoundary drops the media type's other parameters #1600

Problem

MultiPartRepresentation.setBoundary(MediaType, String) drops the media type's other parameters when it already has a boundary parameter. It builds the new media type from the parent type only:

if (mediaType.getParameters().getFirst(PARAMETER_BOUNDARY) != null) {
    result = new MediaType(mediaType.getParent(), PARAMETER_BOUNDARY, boundary);
}

So multipart/mixed; boundary=a; deferSpec=20220824 becomes multipart/mixed; boundary=b, and deferSpec is lost. The instance method setBoundary(String) has the same effect, since it delegates to the static one. When the media type has no boundary yet, the other parameters are kept, so the behavior depends on the input.

Why it matters

Some protocols carry other parameters on multipart types, such as deferSpec for GraphQL incremental delivery and type/start for multipart/related. The new streaming MultipartStreamRepresentation (#1587, #1595) keeps them.

Proposed direction

Replace only the boundary parameter and keep the others in their order. Add a test covering both cases, with and without an existing boundary.

Related

• Add a streaming multipart/mixed representation and reader #1587 / feat: add a streaming multipart representation and reader #1595: streaming multipart/mixed