message: Update flags to string conversion for JSON
diff: @@ -168,7 +168,7 @@ UniValue blockheaderToJSON(const CBlockIndex* blockindex)
  if (pnext)
  result.push_back(Pair("nextblockhash", pnext->GetBlockHash().GetHex()));
  - result.push_back(Pair("flags", strprintf("%s%s", blockindex->IsProofOfStake()? "proof-of-stake" : "proof-of-work")));
  + result.push_back(Pair("flags", strprintf("%s", blockindex->IsProofOfStake()? "proof-of-stake" : "proof-of-work")));
  result.push_back(Pair("proofhash", blockindex->hashProof.GetHex()));
  result.push_back(Pair("modifier", blockindex->nStakeModifier.GetHex()));
  @@ -219,7 +219,7 @@ UniValue blockToJSON(const CBlock& block, const CBlockIndex* blockindex, bool tx
  if (pnext)
  result.push_back(Pair("nextblockhash", pnext->GetBlockHash().GetHex()));
  - result.push_back(Pair("flags", strprintf("%s%s", blockindex->IsProofOfStake()? "proof-of-stake" : "proof-of-work")));
  + result.push_back(Pair("flags", strprintf("%s", blockindex->IsProofOfStake()? "proof-of-stake" : "proof-of-work")));
  result.push_back(Pair("proofhash", blockindex->hashProof.GetHex()));
  result.push_back(Pair("modifier", blockindex->nStakeModifier.GetHex()));
  