Add CurrentBillingCycle field to Modification (braintree-go#236)

What
===
Add CurrentBillingCycle field to Modification.

Why
===
To get access to the field returned in the modification response from the Braintree API.

* Add CHANGELOG entry

Author: Shugyousha

CHANGELOG.md

@@ -8,6 +8,7 @@ IMPROVEMENTS:
 
 * Add TransactionLineItemGateway, TransactionLineItem, TransactionLineItemRequest.
 * Add LineItems to TransactionRequest.
+* Add CurrentBillingCycle to Modification.
 
 ## 0.18.0 (March 2nd, 2018)
 

discount_integration_test.go

@@ -34,6 +34,8 @@ func TestDiscounts(t *testing.T) {
 		t.Fatalf("expected Name to be %s, was %s", "test_discount_name", discount.Name)
 	} else if discount.NeverExpires != true {
 		t.Fatalf("expected NeverExpires to be %t, was %t", true, discount.NeverExpires)
+	} else if discount.CurrentBillingCycle != 0 {
+		t.Fatalf("expected current billing cycle to be %d, was %d", 0, discount.CurrentBillingCycle)
 	} else if discount.Description != "A test discount" {
 		t.Fatalf("expected Description to be %s, was %s", "A test discount", discount.Description)
 	}

modification.go

@@ -16,5 +16,6 @@ type Modification struct {
 	NeverExpires          bool       `xml:"never-expires"`
 	Quantity              int        `xml:"quantity"`
 	NumberOfBillingCycles int        `xml:"number-of-billing-cycles"`
+	CurrentBillingCycle   int        `xml:"current-billing-cycle"`
 	UpdatedAt             *time.Time `xml:"updated_at"`
 }

subscription_integration_test.go

@@ -925,6 +925,9 @@ func TestSubscriptionModifications(t *testing.T) {
 	if x := sub2.Discounts.Discounts[0].NumberOfBillingCycles; x != 2 {
 		t.Fatalf("got %v number of billing cycles on discount, want 2 billing cycles", x)
 	}
+	if x := sub2.Discounts.Discounts[0].CurrentBillingCycle; x != 0 {
+		t.Fatalf("got current billing cycle of %d on discount, want 0", x)
+	}
 
 	// Update AddOn
 	sub3, err := g.Update(ctx, &SubscriptionRequest{