Conflicting Basic Auth Fixes

This is a slight modification of @DamnDam's pull request https://github.com/owncloud/core/pull/6922

This fixes cases where..

1. A browser sends HTTP Basic Auth Headers that don't match a current ownCloud user.
    * Login form submittal is preferred.  HTTP Auth User is fixed with http(s)://user@domain/webroot
2. A browser sends HTTP Basic Auth Headers that do match a current user, then user tries logging out.  Previously it was impossible to logout without clearing your browser's history.
    * HTTP Auth User is overwritten with http(s)://logout@domain/webroot

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Fixes https://github.com/owncloud/core/pull/6922 https://github.com/owncloud/core/issues/4556