providers/digitalocean: compat for changes to API for droplet size

Upstream in the digitalocean API, the size slug became a size
string, breaking pearkes/digitalocean.

https://github.com/pearkes/digitalocean/pull/1

This fixes compatibility for changes to that library.

closes #510

builtin/providers/digitalocean/resource_digitalocean_droplet.go

@@ -296,7 +296,7 @@ func resource_digitalocean_droplet_update_state(
 		s.Attributes["ipv4_address_private"] = droplet.IPV4Address("private")
 	}
 
-	s.Attributes["size"] = droplet.SizeSlug()
+	s.Attributes["size"] = droplet.SizeSlug
 	s.Attributes["status"] = droplet.Status
 
 	return s, nil

builtin/providers/digitalocean/resource_digitalocean_droplet_test.go

@@ -123,8 +123,8 @@ func testAccCheckDigitalOceanDropletAttributes(droplet *digitalocean.Droplet) re
 			return fmt.Errorf("Bad image_slug: %s", droplet.ImageSlug())
 		}
 
-		if droplet.SizeSlug() != "512mb" {
-			return fmt.Errorf("Bad size_slug: %s", droplet.SizeSlug())
+		if droplet.SizeSlug != "512mb" {
+			return fmt.Errorf("Bad size_slug: %s", droplet.SizeSlug)
 		}
 
 		if droplet.RegionSlug() != "nyc3" {
@@ -141,8 +141,8 @@ func testAccCheckDigitalOceanDropletAttributes(droplet *digitalocean.Droplet) re
 func testAccCheckDigitalOceanDropletRenamedAndResized(droplet *digitalocean.Droplet) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 
-		if droplet.SizeSlug() != "1gb" {
-			return fmt.Errorf("Bad size_slug: %s", droplet.SizeSlug())
+		if droplet.SizeSlug != "1gb" {
+			return fmt.Errorf("Bad size_slug: %s", droplet.SizeSlug)
 		}
 
 		if droplet.Name != "baz" {
@@ -160,8 +160,8 @@ func testAccCheckDigitalOceanDropletAttributes_PrivateNetworkingIpv6(droplet *di
 			return fmt.Errorf("Bad image_slug: %s", droplet.ImageSlug())
 		}
 
-		if droplet.SizeSlug() != "1gb" {
-			return fmt.Errorf("Bad size_slug: %s", droplet.SizeSlug())
+		if droplet.SizeSlug != "1gb" {
+			return fmt.Errorf("Bad size_slug: %s", droplet.SizeSlug)
 		}
 
 		if droplet.RegionSlug() != "sgp1" {