btcjson+rpcclient: restore bitcoind compatibility

The PR btcsuite#1594 introduced a change that made the order of parameters
relevant, if one of them is nil. This makes it harder to be backward
compatible with the same JSON message if an existing parameter in
bitcoind was re-purposed to have a different meaning.

btcjson/chainsvrcmds.go

@@ -822,8 +822,8 @@ func NewSearchRawTransactionsCmd(address string, verbose, skip, count *int, vinE
 // SendRawTransactionCmd defines the sendrawtransaction JSON-RPC command.
 type SendRawTransactionCmd struct {
 	HexTx         string
-	AllowHighFees *bool `jsonrpcdefault:"false"`
 	MaxFeeRate    *int32
+	AllowHighFees *bool `jsonrpcdefault:"false"`
 }
 
 // NewSendRawTransactionCmd returns a new instance which can be used to issue a