storagezone: use d.Partial in Update instead of revertUpdateValues

That after a failed update changes to the resource are still applied is a know
bug (hashicorp/terraform-plugin-sdk#476).
It affects attributes that are not retrieved via the Get function and set only
by the provider.
custom_404_file_path is such a field.
As workaround d.Partial() can be called.
This is more simple then the revertUpdateValues() implementation.

internal/provider/resource_storagezone.go

@@ -272,14 +272,11 @@ func resourceStorageZoneUpdate(ctx context.Context, d *schema.ResourceData, meta
 
  updateErr := clt.StorageZone.Update(ctx, id, storageZone)
  if updateErr != nil {
- // if our update failed then revert our values to their original
- // state so that we can run an apply again.
- revertErr := revertUpdateValues(d)
-
- if revertErr != nil {
- return diagsErrFromErr("updating storage zone via API failed", revertErr)
- }
-
+ // The storagezone contains fields /custom_404_file_path) that are only updated by the
+ // provider and not retrieved via ReadContext(). This causes that we run into the bug
+ // https://github.com/hashicorp/terraform-plugin-sdk/issues/476.
+ // As workaround d.Partial(true) is called.
+ d.Partial(true)
  return diagsErrFromErr("updating storage zone via API failed", updateErr)
  }
 
@@ -370,23 +367,6 @@ func storageZoneToResource(sz *bunny.StorageZone, d *schema.ResourceData) error
  return nil
 }
 
-func revertUpdateValues(d *schema.ResourceData) error {
- o, _ := d.GetChange(keyOriginURL)
- if err := d.Set(keyOriginURL, o); err != nil {
- return err
- }
- o, _ = d.GetChange(keyCustom404FilePath)
- if err := d.Set(keyCustom404FilePath, o); err != nil {
- return err
- }
- o, _ = d.GetChange(keyRewrite404To200)
- if err := d.Set(keyRewrite404To200, o); err != nil {
- return err
- }
-
- return nil
-}
-
 // storageZoneFromResource returns a StorageZoneUpdateOptions API type that
 // has fields set to the values in d.
 func storageZoneFromResource(d *schema.ResourceData) *bunny.StorageZoneUpdateOptions {