docs: public HeaderMap put(HttpString headerName, String headerValue) {
          if (headerName == null) {
              throw new IllegalArgumentException("headerName is null");
          }
          if (headerValue == null) {
              remove(headerName);
              return this;
          }
          final HeaderValues headerValues = getOrCreateEntry(headerName);
          headerValues.clear();
          headerValues.add(headerValue);
          return this;
      }