allow setting of a demodulized class name when using STI

If your STI class looks like this:

```ruby
class Company < ActiveRecord::Base
  self.store_full_sti_class = false

  class GoodCo < Company
  end

  class BadCo < Company
  end
end
```

The expectation (which is valid) is that the `type` in the database is saved as
`GoodCo` or `BadCo`. However, another expectation should be that setting `type`
to `GoodCo` would correctly instantiate the object as a `Company::GoodCo`. That
second expectation is what this should fix.

activerecord/CHANGELOG.md

+*   Allow setting of a demodulized class name whenusing single table inheritance.
+
+    *Alex Robbin*
+
 *   Dump indexes in `create_table` instead of `add_index`.
 
     If the adapter supports indexes in create table, generated SQL is

activerecord/lib/active_record/inheritance.rb

@@ -198,14 +198,16 @@ def subclass_from_attributes?(attrs)
       def subclass_from_attributes(attrs)
         subclass_name = attrs.with_indifferent_access[inheritance_column]
 
-        if subclass_name.present? && subclass_name != self.name
-          subclass = subclass_name.safe_constantize
+        if subclass_name.present?
+          subclass = find_sti_class(subclass_name)
 
-          unless descendants.include?(subclass)
-            raise ActiveRecord::SubclassNotFound.new("Invalid single-table inheritance type: #{subclass_name} is not a subclass of #{name}")
-          end
+          if subclass.name != self.name
+            unless descendants.include?(subclass)
+              raise ActiveRecord::SubclassNotFound.new("Invalid single-table inheritance type: #{subclass_name} is not a subclass of #{name}")
+            end
 
-          subclass
+            subclass
+          end
         end
       end
     end

activerecord/test/cases/inheritance_test.rb

@@ -217,6 +217,14 @@ def test_new_with_complex_inheritance
     assert_nothing_raised { Client.new(type: 'VerySpecialClient') }
   end
 
+  def test_new_without_storing_full_sti_class
+    without_store_full_sti_class do
+      Company::SpecialCo # force autoloading
+      item = Company.new(type: 'SpecialCo')
+      assert_instance_of Company::SpecialCo, item
+    end
+  end
+
   def test_new_with_autoload_paths
     path = File.expand_path('../../models/autoloadable', __FILE__)
     ActiveSupport::Dependencies.autoload_paths << path

activerecord/test/models/company.rb

@@ -26,6 +26,9 @@ def arbitrary_method
   def private_method
     "I am Jack's innermost fears and aspirations"
   end
+
+  class SpecialCo < Company
+  end
 end
 
 module Namespaced