def check_pow(nonce=nil)
      logger.debug "checking pow", block: full_hash_hex[0,8]
      Miner.check_pow(number, mining_hash, mixhash, nonce || self.nonce, difficulty)
    end
def check_pow(nonce=nil)
      logger.debug "checking pow", block: full_hash_hex[0,8]
      Miner.check_pow(number, mining_hash, mixhash, nonce || self.nonce, difficulty)
    end
Check if the proof-of-work of the block is valid.

 @param nonce [String] if given the proof of work function will be
   evaluated with this nonce instead of the one already present in the
   header

 @return [Bool]