def sign(plain_text, jws = nil, header = nil)
      jws ||= signer
      return JOSE::JWS.sign(self, plain_text, jws, header)
    end
def sign(plain_text, jws = nil, header = nil)
      jws ||= signer
      return JOSE::JWS.sign(self, plain_text, jws, header)
    end
Signs the `plain_text` using the `jwk` and the default signer algorithm `jws` for the key type.

 @see JOSE::JWS.sign
 @param [String] plain_text
 @param [JOSE::JWS] jws
 @param [JOSE::Map] header
 @return [JOSE::SignedMap]