[4.0] Secure string generator acts like string generator.

Hi,

I want to use your bundle for my project (Symfony 4.3)

In my entity:
```
/**
     * @ORM\Column(type="string", length=255, unique=true)
     * @Generate(generator="secure_string")
     */
    private $codeReset;
```
and generated code is `ZieI7WRG`. Of course, I can use options, but doesn't look as defaults.

Also, no new generated string on update. 

Thank you,

Paul


