Fixed plugin ordering for live plugins.

Refs #3637

When publishing a page, the draft plugins are copied to new 'live' versions of themselves. The code was incorrectly overwriting the position of the plugins. This change means the position remains identical to the position of the draft plugin, which is the only correct value.

This pull request is incomplete because it lacks a test case demonstrating that the issue exists as well as proving that this change fixes it. I've had challenges getting a test case to exercise the appropriate code paths since setting up the test data appears to require front-end editing and can't be triggered via the api as the current `test_plugin_order` does.