code: public JWSHeader setKeyId(String keyId) {
          this.keyId = keyId;
          this.put(HeaderConstants.KEY_ID, keyId);
          return this;
      }
docs: Sets the key ID header parameter that is a hint indicating which specific key owned by the
  signer should be used to validate the digital signature or {@code null} for none.
  
  <p>
  Overriding is only supported for the purpose of calling the super implementation and changing
  the return type, but nothing else.
  </p>