…set to "true" (nieuwenhuisen)

This PR was merged into the 3.4 branch.

Discussion
----------

[SecurityBundle] Passwords are not encoded when algorithm set to "true"

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #34725
| License       | MIT
| Doc PR        | -

If the algorithm is set to `true`, password will be encode as plain password.

```
security:
    encoders:
        App\User\User:
            algorithm: true
```

The reason for this is the not strict comparison of php switches.

```
switch ($config['algorithm']) {
            case 'plaintext':
}
```

`true == 'plaintext'` is `true`, so the first case is hit. My first solution was to cast the algorithm to a string, to prevent this. After some feedback I have catch this problem earlier and does not allow true as valid value to the algorithm option.

Ps. This is my first PR for Symfony, any feedback is welcome :-)!

Commits
-------

83a5517 [SecurityBundle] Passwords are not encoded when algorithm set to \"true\"