rpc: wrap calculatenetworkfee result in a structure

C#:
{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "networkfee": "1185120"
  }
}

Go:
{
  "id": 2,
  "jsonrpc": "2.0",
  "result": 1185120
}

Thanks @csmuller for finding it.

pkg/rpc/server/server.go

@@ -630,7 +630,7 @@ func (s *Server) calculateNetworkFee(reqParams request.Params) (interface{}, *re
 	}
 	fee := s.chain.GetPolicer().FeePerByte()
 	netFee += int64(size) * fee
-	return netFee, nil
+	return result.NetworkFee{Value: netFee}, nil
 }
 
 // getApplicationLog returns the contract log based on the specified txid or blockid.