MultipartFormDataOutput limits 1 part per name

I would like to be able to specify more than one OutputPart for a given name in MultipartFormDataOutput.

I'm trying to build a RESTEasy client to an internal legacy service. The service expects more than one InputPart for a single part name, which works fine because MultipartFormDataInputImpl uses List<InputPart> as the value in the Map that it generates (changed here: https://github.com/resteasy/Resteasy/commit/33225ade62add7592a595d816a160b82f9c02c26). However, the client fails to add more than one part for a given name because MultipartFormDataOutput uses LinkedHashMap<String, OutputPart>.