feature/support merchant advice code and missing 3DS fields (#88)

## Description
- support merchant advice 
- support merchant advice code
- support missing 3DS fields in charge 
- update unit test

Author: tyler-techcombine

charge.go

@@ -39,6 +39,10 @@ type Charge struct {
 	Source    *Source                `json:"source"`
 	Metadata  map[string]interface{} `json:"metadata"`
 	ExpiresAt time.Time              `json:"expires_at"`
+
+	MerchantAdvice     string   `json:"merchant_advice"`
+	MerchantAdviceCode string   `json:"merchant_advice_code"`
+	Missing3DSFields   []string `json:"missing_3ds_fields"`
 }
 
 type TransactionIndicator string

operations/charge_test.go

@@ -315,3 +315,15 @@ func TestRetrieveCharge_HasExpiredAt(t *testing.T) {
 
 	r.Equal(t, "2020-07-01 03:53:01 +0000 UTC", charge.ExpiresAt.String())
 }
+
+func TestRetrieveChargeHas3DSFields(t *testing.T) {
+	const ChargeID = "chrg_test_4yq7duw15p9hdrjp8oq"
+	client := testutil.NewFixedClient(t)
+
+	charge := &omise.Charge{}
+	client.MustDo(charge, &RetrieveCharge{ChargeID})
+
+	r.Equal(t, "additional authentication required", charge.MerchantAdvice)
+	r.Equal(t, "03", charge.MerchantAdviceCode)
+	r.Equal(t, []string{"phone_number", "address"}, charge.Missing3DSFields)
+}

testdata/fixtures/api.omise.co/charges/chrg_test_4yq7duw15p9hdrjp8oq-get.json

@@ -54,5 +54,8 @@
   },
   "customer": "cust_test_4yq6txdpfadhbaqnwp3",
   "ip": null,
-  "created_at": "2015-01-15T05:00:29Z"
+  "created_at": "2015-01-15T05:00:29Z",
+  "merchant_advice": "additional authentication required",
+  "merchant_advice_code": "03", 
+  "missing_3ds_fields": ["phone_number", "address"]
 }