docs: @Override
      public Client header(String name, Object... values) {
          if (values == null) {
              throw new IllegalArgumentException();
          }
          if (HttpHeaders.CONTENT_TYPE.equals(name)) {
              if (values.length > 1) {
                  throw new IllegalArgumentException("Content-Type can have a single value only");
              }
              type(convertParamValue(values[0], null));
          } else {
              for (Object o : values) {
                  possiblyAddHeader(name, convertParamValue(o, null));
              }
          }
          return this;
      }