Merge branch 'hash_with_indifferent_access_fix'

The problem:
Accessing a HashWithIndifferentAccess does not return the the same object
that is stored in the hash (i.e. equal?) causing unexpected results:

    hash = HashWithIndifferentAccess.new {|h, k| h[k] = []}
    hash[:a] << 1  # => [1]
    hash[:a]       # => [], expected [1]

The cause:
When a block is provided to generate default values the generated values
are duped if they are arrays. The duped value is stored in the hash but
the original value is returned when the hash is accessed.

The fix:
The duping is there for allowing frozen arrays containing hashes to be
modified. The fix restricts the duping to this case. Note that if default
function generates a frozen array an error will be raised on assignment
before and after the patch.

Closes #3811

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -164,7 +164,8 @@ def convert_value(value)
         if value.is_a? Hash
           value.nested_under_indifferent_access
         elsif value.is_a?(Array)
-          value.dup.replace(value.map { |e| convert_value(e) })
+          value = value.dup if value.frozen?
+          value.map! { |e| convert_value(e) }
         else
           value
         end

activesupport/test/core_ext/hash_ext_test.rb

@@ -457,6 +457,13 @@ def test_indifferent_to_hash
     assert_equal '1234', roundtrip.default
   end
 
+  def test_lookup_returns_the_same_object_that_is_stored_in_hash_indifferent_access
+    hash = HashWithIndifferentAccess.new {|h, k| h[k] = []}
+    hash[:a] << 1
+
+    assert_equal [1], hash[:a]
+  end
+
   def test_indifferent_hash_with_array_of_hashes
     hash = { "urls" => { "url" => [ { "address" => "1" }, { "address" => "2" } ] }}.with_indifferent_access
     assert_equal "1", hash[:urls][:url].first[:address]