correct issue #684 (#686)

Author: Sébastien GLON

google/resource_google_service_account_key.go

@@ -43,7 +43,7 @@ func resourceGoogleServiceAccountKey() *schema.Resource {
 			},
 			"public_key_type": &schema.Schema{
 				Type:         schema.TypeString,
-				Default:      "X509_PEM",
+				Default:      "TYPE_X509_PEM_FILE",
 				Optional:     true,
 				ForceNew:     true,
 				ValidateFunc: validation.StringInSlice([]string{"TYPE_NONE", "TYPE_X509_PEM_FILE", "TYPE_RAW_PUBLIC_KEY"}, false),