Fix rails#3737 AS::expand_cache_key generates wrong key in certain situations

`cache_key` method is never called when the argument is a 1-element array
with something that responds to `cache_key`

Author: exviva

activesupport/lib/active_support/cache.rb

@@ -81,16 +81,10 @@ def self.expand_cache_key(key, namespace = nil)
       end
 
       expanded_cache_key <<
-        if key.respond_to?(:cache_key)
-          key.cache_key
-        elsif key.is_a?(Array)
-          if key.size > 1
-            key.collect { |element| expand_cache_key(element) }.to_param
-          else
-            key.first.to_param
-          end
-        elsif key
-          key.to_param
+        case
+        when key.respond_to?(:cache_key) then key.cache_key
+        when key.is_a?(Array)            then key.map { |element| expand_cache_key(element) }.to_param
+        when key                         then key.to_param
         end.to_s
 
       expanded_cache_key

activesupport/test/caching_test.rb

@@ -12,10 +12,10 @@ def test_expand_cache_key_with_rails_cache_id
     begin
       ENV['RAILS_CACHE_ID'] = 'c99'
       assert_equal 'c99/foo', ActiveSupport::Cache.expand_cache_key(:foo)
-      assert_equal 'c99/foo', ActiveSupport::Cache.expand_cache_key([:foo])
+      assert_equal 'c99/c99/foo', ActiveSupport::Cache.expand_cache_key([:foo])
       assert_equal 'c99/c99/foo/c99/bar', ActiveSupport::Cache.expand_cache_key([:foo, :bar])
       assert_equal 'nm/c99/foo', ActiveSupport::Cache.expand_cache_key(:foo, :nm)
-      assert_equal 'nm/c99/foo', ActiveSupport::Cache.expand_cache_key([:foo], :nm)
+      assert_equal 'nm/c99/c99/foo', ActiveSupport::Cache.expand_cache_key([:foo], :nm)
       assert_equal 'nm/c99/c99/foo/c99/bar', ActiveSupport::Cache.expand_cache_key([:foo, :bar], :nm)
     ensure
       ENV['RAILS_CACHE_ID'] = nil
@@ -50,6 +50,14 @@ def key.cache_key
     assert_equal 'foo_key', ActiveSupport::Cache.expand_cache_key(key)
   end
 
+  def test_array_with_something_that_responds_to_cache_key
+    key = 'foo'
+    def key.cache_key
+      :foo_key
+    end
+    assert_equal 'foo_key', ActiveSupport::Cache.expand_cache_key([key])
+  end
+
 end
 
 class CacheStoreSettingTest < ActiveSupport::TestCase