add query (?) method to store_accessor

The accessor methods generated for store or store_accessor attributes
lack a ? method the normal active record attributes do have.
This add that method by verifying if the value is not nil?.

    class TestModel < ActiveRecord::Base
      store_accessor :data, :width, :height
    end

    test_model = TestModel.new(data: {width: 5})
    test_model.width?   #=> true
    test_model.height?  #=> false