Explicitly handle error 404 the API can return.

I'd forgotten about this case, but it's used when the user hasn't set
any Image Optimizer settings, and it's useful for terraform to be able
to easily detect this case.

I assume other users of this API will also appreciate this explicit
error handling, as opposed to having to check for error 404 themselves.

fastly/image_optimizer_default_settings.go

@@ -87,6 +87,8 @@ type UpdateImageOptimizerDefaultSettingsInput struct {
 }
 
 // GetImageOptimizerDefaultSettings retrives the current Image Optimizer default settings on a given service version.
+//
+// Returns (nil, nil) if no default settings are set.
 func (c *Client) GetImageOptimizerDefaultSettings(i *GetImageOptimizerDefaultSettingsInput) (*ImageOptimizerDefaultSettings, error) {
 	if i.ServiceID == "" {
 		return nil, ErrMissingServiceID
@@ -99,6 +101,12 @@ func (c *Client) GetImageOptimizerDefaultSettings(i *GetImageOptimizerDefaultSet
 
 	resp, err := c.Get(path, nil)
 	if err != nil {
+		if herr, ok := err.(*HTTPError); ok {
+			if herr.StatusCode == 404 {
+				// API endpoint returns 404 for services without Image Optimizer settings set.
+				return nil, nil
+			}
+		}
 		return nil, err
 	}
 	defer resp.Body.Close()