make Array#cache_key work with frozen arrays

test-plan (script/console):
 - make an Array (ary = [])
 - freeze it (ary.freeze)
 - try and get a cache_key from it (ary.cache_key)
 - should not raise an error

Change-Id: I2646440d7d48d2cea16140cd15c8bef56cd3e063
Reviewed-on: https://gerrit.instructure.com/18717
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>

Author: lukfugl

lib/ext/array.rb

@@ -32,7 +32,13 @@ def to_atom
   end
 
   def cache_key
-    @cache_key ||= self.collect{|element| ActiveSupport::Cache.expand_cache_key(element) }.to_param
+    if @cache_key
+      @cache_key
+    else
+      value = self.collect{|element| ActiveSupport::Cache.expand_cache_key(element) }.to_param
+      @cache_key = value unless self.frozen?
+      value
+    end
   end
 
   def once_per(&block)

spec/lib/ext/array_spec.rb

@@ -0,0 +1,27 @@
+#
+# Copyright (C) 2011 Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+require File.expand_path(File.dirname(__FILE__) + '/../../spec_helper.rb')
+
+describe "Array#cache_key" do
+  it "should work with frozen arrays" do
+    array = [1, 2, 3]
+    array.freeze
+    array.cache_key.should == '1/2/3'
+  end
+end