Edit in Word does not work when a custom authenticator is configured

The "Edit in Word" feature currently causes authentication to be run twice. The first is done by Confluence's security filter and properly uses the custom authenticator. The second call wrongly uses the DefaultAuthenticator and not the custom authenticator due CONF-20365.

There are 2 issues here, either of which will fix this and allow it to work again, but both of which should be fixed:

1. CONF-20365 The custom authenticator is not respected
2. The second authentication is redundant.