DeleteCollection: Make params a struct

Rather than accepting a naked string, accept a DeleteCollectionParams
struct so that new optional parameters can be added in the future
without breaking the API.

collection.go

@@ -29,6 +29,12 @@ type (
 		Alias string `json:"alias"`
 		Title string `json:"title"`
 	}
+
+	// DeleteCollectionParams holds the parameters required to delete a
+	// collection.
+	DeleteCollectionParams struct {
+		Alias string `json:"-"`
+	}
 )
 
 // CreateCollection creates a new collection, returning a user-friendly error
@@ -140,8 +146,8 @@ func (c *Client) GetUserCollections() (*[]Collection, error) {
 // anonymous.
 //
 // See https://developers.write.as/docs/api/#delete-a-collection.
-func (c *Client) DeleteCollection(alias string) error {
-	endpoint := "/collections/" + alias
+func (c *Client) DeleteCollection(p *DeleteCollectionParams) error {
+	endpoint := "/collections/" + p.Alias
 	env, err := c.delete(endpoint, nil /* data */)
 	if err != nil {
 		return err

collection_test.go

@@ -71,7 +71,8 @@ func TestCreateAndDeleteCollection(t *testing.T) {
 		t.Fatalf("Unable to create collection %q: %v", alias, err)
 	}
 
-	if err := wac.DeleteCollection(c.Alias); err != nil {
+	p := &DeleteCollectionParams{Alias: c.Alias}
+	if err := wac.DeleteCollection(p); err != nil {
 		t.Fatalf("Unable to delete collection %q: %v", alias, err)
 	}
 }
@@ -81,7 +82,8 @@ func TestDeleteCollectionUnauthenticated(t *testing.T) {
 
 	now := time.Now().Unix()
 	alias := fmt.Sprintf("test-collection-does-not-exist-%v", now)
-	err := wac.DeleteCollection(alias)
+	p := &DeleteCollectionParams{Alias: alias}
+	err := wac.DeleteCollection(p)
 	if err == nil {
 		t.Fatalf("Should not be able to delete collection %q unauthenticated.", alias)
 	}