docs: public void setDeferredTrailer(HeaderKeys hdr, HttpTrailerGenerator htg) {
          if (tc.isDebugEnabled()) {
              Tr.debug(tc, "setDeferredTrailer(HeaderKeys): " + hdr);
          }
          if (null == hdr) {
              throw new IllegalArgumentException("Null header name");
          }
          if (null == htg) {
              throw new IllegalArgumentException("Null value generator");
          }
  
          this.knownTGs.put(hdr, htg);
      }