code: public void addFieldPart(String name, ContentProvider content, HttpFields fields) {
          addPart(new Part(name, null, "text/plain", content, fields));
      }
docs: <p>Adds a field part with the given {@code name} as field name, and the given
  {@code content} as part content.</p>
  
  @param name    the part name
  @param content the part content
  @param fields  the headers associated with this part