code: protected Request.Builder setBody(Request.Builder builder) {
          if (body != null) {
              RequestBody requestBody = null;
              if (requestBody == null) {
                  FormBody.Builder fbBuilder = new FormBody.Builder();
                  Iterator<String> itr = body.keySet().iterator();
                  while (itr.hasNext()) {
                      String key = itr.next();
                      fbBuilder.add(key, (String) body.get(key));
                  }
                  requestBody = fbBuilder.build();
              }
  
              if ("PUT".equalsIgnoreCase(method))
                  builder.put(requestBody);
              else if ("POST".equalsIgnoreCase(method))
                  builder.post(requestBody);
          }
          return builder;
      }
docs: set request body