Pass returnpage to authentication method selector form

When more then one log-in methods are specified, then log-in screen is like:

![Image 1](https://f.cloud.github.com/assets/628801/274671/9e86e842-906d-11e2-8207-98c8a766681d.png)

If the user clicks on one, then form is refreshed and passed parameter "returnpage" is lost.

This PR resolves this bug.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | no
| License       | MIT
| Doc PR        | no