eth/tracers: make transaction trace response compatible with geth

The current transactionHash field in transaction trace response should be named
txHash to be compatible with go-ethereum. This commit adds txHash instead of
replacing the current transactionHash field and make them have the same value.
This can help to avoid breaking change for current users of these methods since
users are unlikely to use strict DisallowUnknownFields when decoding the json
response.

eth/tracers/api.go

@@ -184,11 +184,32 @@ type StdTraceConfig struct {
 
 // txTraceResult is the result of a single transaction trace.
 type txTraceResult struct {
-	TransactionHash common.Hash `json:"transactionHash,omitempty"`
+	TransactionHash common.Hash `json:"transactionHash"`
 	Result          interface{} `json:"result,omitempty"` // Trace results produced by the tracer
 	Error           string      `json:"error,omitempty"`  // Trace failure produced by the tracer
 }
 
+// Create a response that is compatible with go-ethereum
+// with txHash field but still keep transactionHash field
+// to avoid breaking change for current user.
+// FIXME: Remove this function and change transactionHash
+// to txHash after everyone has changed to use the new one.
+func (result txTraceResult) MarshalJSON() ([]byte, error) {
+	newResult := struct {
+		TxHash          common.Hash `json:"txHash"`
+		TransactionHash common.Hash `json:"transactionHash"`
+		Result          interface{} `json:"result,omitempty"`
+		Error           string      `json:"error,omitempty"`
+	}{
+		TxHash:          result.TransactionHash,
+		TransactionHash: result.TransactionHash,
+		Result:          result.Result,
+		Error:           result.Error,
+	}
+
+	return json.Marshal(&newResult)
+}
+
 // internalAndAccountResult is the result of a single transaction trace.
 type internalAndAccountResult struct {
 	InternalTxs   []*txTraceResult `json:"internalTxs,omitempty"` // Trace results produced by the tracer