Add Devise case insensitive keys integration

**As a** developer 
**In order to** be able to use the Devise's **case-insensitive keys** feature during token authentication 
**I want** _Simple Token Authentication_ to take it into account when performing authentication 

See #137 and [the Devise **case-insensitive keys**feature documentation](https://github.com/plataformatec/devise/blob/v3.4.1/lib/generators/templates/devise.rb#L45-L48).