Refactor token creation

[MaicolBen]

I didn't know if I should include @resource.save! in the new method, also method name suggestions are appreciated.

[KelseyDH]

It might be a good idea to avoid the use of ||= if we can, as apparently the ||= operator may not be thread safe even in MRI Ruby.

[MaicolBen]

Thanks, but that applies to instance variables, you can't share two method variables unless you create a thread inside that method.

[zachfeldman]

@MaicolBen sorry I haven't seen this yet! I approve of this line of thought and your changes. Fix your conflicts and we should be good to merge!

[lynndylanhurley]

Very nice. Thanks @MaicolBen!!

[Evan-M]

Are the two instance methods in DeviseTokenAuth::Concern::User (#create_new_auth_token and #create_token) duplicate logic? Could they be refactored further to DRY up the logic?

Additionally, I would think that #build_auth_header and #update_auth_header in the DeviseTokenAuth::Concern::User model concern, are perhaps not a proper separation of concerns (MVC). Does a User model (or other Devise resource) need to know about request/response headers?

UPDATE: I realize that this comment is not as clear as I had intended. Thus I have created a new PR #1098.