transaction: unmarshal Witness properly

Both verification and invocation scripts need to
be unmarshaled from hex.
Also fix failing RPC tests: block contains non-pointer
`transaction.Witness` field and (*Witness).MarshalJSON method
is not called.

pkg/core/transaction/transaction_test.go

@@ -185,6 +185,10 @@ func TestMarshalUnmarshalJSON(t *testing.T) {
 		ScriptHash: util.Uint160{7, 8, 9, 10},
 		Position:   13,
 	}}
+	tx.Scripts = []Witness{{
+		InvocationScript:   []byte{5, 3, 1},
+		VerificationScript: []byte{2, 4, 6},
+	}}
 	data, err := json.Marshal(tx)
 	require.NoError(t, err)
 

pkg/core/transaction/witness.go

@@ -28,7 +28,7 @@ func (w *Witness) EncodeBinary(bw *io.BinWriter) {
 }
 
 // MarshalJSON implements the json marshaller interface.
-func (w *Witness) MarshalJSON() ([]byte, error) {
+func (w Witness) MarshalJSON() ([]byte, error) {
 	data := map[string]string{
 		"invocation":   hex.EncodeToString(w.InvocationScript),
 		"verification": hex.EncodeToString(w.VerificationScript),
@@ -37,6 +37,20 @@ func (w *Witness) MarshalJSON() ([]byte, error) {
 	return json.Marshal(data)
 }
 
+// UnmarshalJSON implements json.Unmarshaler interface.
+func (w *Witness) UnmarshalJSON(data []byte) error {
+	m := map[string]string{}
+	err := json.Unmarshal(data, &m)
+	if err != nil {
+		return err
+	}
+	if w.InvocationScript, err = hex.DecodeString(m["invocation"]); err != nil {
+		return err
+	}
+	w.VerificationScript, err = hex.DecodeString(m["verification"])
+	return err
+}
+
 // ScriptHash returns the hash of the VerificationScript.
 func (w Witness) ScriptHash() util.Uint160 {
 	return hash.Hash160(w.VerificationScript)