Unexpected redirect after login

### Steps to Reproduce

1. Start example Application and open localhost:8080
2. Navigate to non existing page e.g localhost:8080/blah, you will be redirected the Login Page (as expected)
3. Don't login, navigate to start page (localhost:8080)
4. Click on grails.plugin.springsecurity.LoginController
5. Login with root:root
6. Now you are on the non existing page, instead of the expected start page

### Expected Behavior

The user should be redirected to the landing page.

### Actual Behaviour

You will be redirected to the previous non existing page (e.g. localhost:8080/blah).

### Environment Information

- **Operating System**: Arch
- **GORM Version**: 6.1.6
- **Grails Version (if using Grails):** 3.3.6
- **JDK Version:** 8
- **Spring Security Core Version**: 3.2.3

### Example Application
Fresh grails app with spring security core and s2-quickstart run. Use root:root will be created on start.

https://github.com/bwollmer/LoginRedirectTest

Reproduce the flow in the linked LoginRedirectTest application, starting with grails.plugin.springsecurity.LoginController and the described localhost:8080 steps. Verify that logging in from the start page after visiting /blah leads to the landing page rather than the nonexistent URL; use that behavior as the completion check.

Written by the indexing model from the issue text.