added test

config/initializers/monkey_patches.rb

@@ -1,3 +1,11 @@
+unless Array.respond_to?(:average)
+  class Array
+    def average
+      inject(:+) / length unless blank?
+    end
+  end
+end
+
 unless Array.respond_to?(:to_h)
   class Array
     def to_h