Change the "selected?" behavior in the multi-enums mode

DESC: Change

1. Return true if stored keys include all given keys, otherwise false
is returned. For example:

user.roles = [:user, :manager]
user.selected?(:user) # => true
user.selected?([:user, :manager]) # => true
user.selected?(:admin) # => false
user.selected?([:user, :admin]) # => false

Please reference the spec for more examples.