use Strong Parameters on User model user_params method

[lipedjow]

This PR suggests 2 small improvements:

• always return ActionParam object on UsersController user_params method (even if empty)
• use Rails Strong Parameters to permit default user params instead of deleting params entries

this way we can easily override user_params method to add new custom fields for User model

let me know if it makes sense to add a configuration entry for changing default new User parameters, for example:
as default: Clearance.configuration.permit_user_parameters = [:email, :password]

we could customize it:
Clearance.configuration.permit_user_parameters = [:name, :email, :password]

and default permit call would looks like:
params.fetch(Clearance.configuration.user_parameter, {}).permit(*Clearance.configuration.permit_user_parameters)

[lipedjow]

ref #542