Correctly determine if an attribute is uninitialized

In real usage, we give the builder a types hash with a default value of
`Type::Value.new`. This means we need to explicitly check for the key,
rather than the truthiness of the type to determine if it's a known but
uninitialized attribute

activerecord/lib/active_record/attribute_set/builder.rb

@@ -65,7 +65,7 @@ def assign_default_proc
 
         if values.key?(name)
           hash[name] = Attribute.from_database(name, values[name], type)
-        elsif type
+        elsif types.key?(name)
           hash[name] = Attribute.uninitialized(name, type)
         end
       end

activerecord/test/cases/attribute_set_test.rb

@@ -109,7 +109,15 @@ class AttributeSetTest < ActiveRecord::TestCase
 
     test "fetch_value returns nil for unknown attributes" do
       attributes = attributes_with_uninitialized_key
-      assert_nil attributes.fetch_value(:wibble)
+      assert_nil attributes.fetch_value(:wibble) { "hello" }
+    end
+
+    test "fetch_value returns nil for unknown attributes when types has a default" do
+      types = Hash.new(Type::Value.new)
+      builder = AttributeSet::Builder.new(types)
+      attributes = builder.build_from_database
+
+      assert_nil attributes.fetch_value(:wibble) { "hello" }
     end
 
     test "fetch_value uses the given block for uninitialized attributes" do