require 'active_support/core_ext' (#361)
require 'active_support/core_ext' (#361)

* require 'active_support/core_ext'

`blank?' for array needs  require 'active_support/core_ext'

* require 'active_support' as well

ref: https://stackoverflow.com/questions/37850575/undefined-method-blank-for-123string-nomethoderror?

Co-authored-by: peter scholz <e255681030e4edf794d83bec236f7a9864e36431@gmail.com>
LeFnord/grape-entity,ruby-grape/grape-entity