docs: public void removeDeferredTrailer(HeaderKeys hdr) {
          if (tc.isDebugEnabled()) {
              Tr.debug(tc, "removeDeferredTrailer(HeaderKeys): " + hdr);
          }
          if (null == hdr) {
              throw new IllegalArgumentException("Null header name");
          }
          this.knownTGs.remove(hdr);
      }