transaction: fix asset type JSONization

It's stringy in C#.

pkg/core/transaction/asset_type.go

@@ -1,5 +1,10 @@
 package transaction
 
+import (
+	"errors"
+	"fmt"
+)
+
 // AssetType represents a NEO asset type.
 type AssetType uint8
 
@@ -14,3 +19,68 @@ const (
 	Invoice        AssetType = DutyFlag | 0x18
 	Token          AssetType = CreditFlag | 0x20
 )
+
+// String implements Stringer interface.
+func (a AssetType) String() string {
+	switch a {
+	case CreditFlag:
+		return "CreditFlag"
+	case DutyFlag:
+		return "DutyFlag"
+	case GoverningToken:
+		return "GoverningToken"
+	case UtilityToken:
+		return "UtilityToken"
+	case Currency:
+		return "Currency"
+	case Share:
+		return "Share"
+	case Invoice:
+		return "Invoice"
+	case Token:
+		return "Token"
+	default:
+		return fmt.Sprintf("Unknonwn (%d)", a)
+
+	}
+}
+
+// MarshalJSON implements the json marshaller interface.
+func (a AssetType) MarshalJSON() ([]byte, error) {
+	return []byte(`"` + a.String() + `"`), nil
+}
+
+// UnmarshalJSON implements the json.Unmarshaler interface.
+func (a *AssetType) UnmarshalJSON(data []byte) error {
+	l := len(data)
+	if l < 2 || data[0] != '"' || data[l-1] != '"' {
+		return errors.New("wrong format")
+	}
+	var err error
+	*a, err = AssetTypeFromString(string(data[1 : l-1]))
+	return err
+}
+
+// AssetTypeFromString converts string into AssetType.
+func AssetTypeFromString(s string) (AssetType, error) {
+	switch s {
+	case "CreditFlag":
+		return CreditFlag, nil
+	case "DutyFlag":
+		return DutyFlag, nil
+	case "GoverningToken":
+		return GoverningToken, nil
+	case "UtilityToken":
+		return UtilityToken, nil
+	case "Currency":
+		return Currency, nil
+	case "Share":
+		return Share, nil
+	case "Invoice":
+		return Invoice, nil
+	case "Token":
+		return Token, nil
+	default:
+		return 0, errors.New("bad asset type")
+	}
+}

pkg/rpc/client/rpc_test.go

@@ -178,7 +178,7 @@ var rpcClientTestCases = map[string][]rpcClientTestCase{
 			invoke: func(c *Client) (interface{}, error) {
 				return c.GetAssetState(util.Uint256{})
 			},
-			serverResponse: `{"id":1,"jsonrpc":"2.0","result":{"id":"0xc56f33fc6ecfcd0c225c4ab356fee59390af8560be0e930faebe74a6daff7c9b","type":0,"name":"NEO","amount":"100000000","available":"100000000","precision":0,"owner":"00","admin":"Abf2qMs1pzQb8kYk9RuxtUb9jtRKJVuBJt","issuer":"AFmseVrdL9f9oyCzZefL9tG6UbvhPbdYzM","expiration":4000000,"frozen":false}}`,
+			serverResponse: `{"id":1,"jsonrpc":"2.0","result":{"id":"0xc56f33fc6ecfcd0c225c4ab356fee59390af8560be0e930faebe74a6daff7c9b","type":"GoverningToken","name":"NEO","amount":"100000000","available":"100000000","precision":0,"owner":"00","admin":"Abf2qMs1pzQb8kYk9RuxtUb9jtRKJVuBJt","issuer":"AFmseVrdL9f9oyCzZefL9tG6UbvhPbdYzM","expiration":4000000,"frozen":false}}`,
 			result: func(c *Client) interface{} {
 				return &result.AssetState{
 					ID:         core.GoverningTokenID(),