Introduce test for array destruction

It was discovered that the concat array modifies the arrays passed to it
as an argument as a side effect. This test will ensure that doesn't
happen again.

spec/functions/concat_spec.rb

@@ -27,4 +27,9 @@ describe "the concat function" do
     expect(result).to(eq(['1','2','3',['4','5'],'6']))
   end
 
+  it "should leave the original array intact" do
+    array_original = ['1','2','3']
+    result = scope.function_concat([array_original,['4','5','6']])
+    array_original.should(eq(['1','2','3']))
+  end
 end