message: fix block.difficulty for PoS
diff: @@ -186,6 +186,7 @@ std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock(const CScript& sc
  if(fProofOfStake)
  txProofTime &= ~STAKE_TIMESTAMP_MASK;
  pblock->nTime = txProofTime;
  + pblock->nBits = GetNextWorkRequired(pindexPrev, pblock, chainparams.GetConsensus(),fProofOfStake);
  const int64_t nMedianTimePast = pindexPrev->GetMedianTimePast();
  nLockTimeCutoff = (STANDARD_LOCKTIME_VERIFY_FLAGS & LOCKTIME_MEDIAN_TIME_PAST)
  