Do not set Spree.user_class after initialize if already set

[kennyadsl]

Currently, when this class is set with a custom value using an initializer in the host application, it is not having any effect because this line is always overwriting the custom value with the default one.

Ref: #169

[tvdeyen]

Is this gem meant to be used when a custom user class is defined? I think it was never meant like this, but was meant to ship the Spree::User class.

If this fix is the only that is needed than I am ok, but what about the migrations it ships? Should we make them conditional as well?

We really should think about the purpose of this gem before making changes like this.

[kennyadsl]

@tvdeyen I think you are right. I supposed (probably wrongly) that the user that reported the issue was using it with an existing "devise" user class. I'll ask for more info in the issue itself.

[kennyadsl]

Closing this one. The issue reported has more to do with solidus_reviews having solidus_auth_devise as a dependency, not allowing people to use it with a custom authentication system.

This is now fixed and this extension still is meant to be used only with Spree::User as Spree.user_class.