Fix select tag helper used with Enumerable choices

Allows a custom object implementing Enumerable to be used as the choices
parameter for a select tag, which previously wasn't possible due to the
call to `empty?` on the choices (which isn't implemented on Enumerable).

actionview/lib/action_view/helpers/tags/select.rb

@@ -33,7 +33,7 @@ module ActionView
           #   [nil, []]
           #   { nil => [] }
           def grouped_choices?
-            !@choices.empty? && @choices.first.respond_to?(:last) && Array === @choices.first.last
+            !@choices.blank? && @choices.first.respond_to?(:last) && Array === @choices.first.last
           end
       end
     end

actionview/test/template/form_options_helper_test.rb

@@ -6,6 +6,15 @@ class Map < Hash
   end
 end
 
+class CustomEnumerable
+  include Enumerable
+
+  def each
+    yield "one"
+    yield "two"
+  end
+end
+
 class FormOptionsHelperTest < ActionView::TestCase
   tests ActionView::Helpers::FormOptionsHelper
 
@@ -904,6 +913,14 @@ class FormOptionsHelperTest < ActionView::TestCase
     )
   end
 
+  def test_select_with_enumerable
+    @post = Post.new
+    assert_dom_equal(
+      "<select id=\"post_category\" name=\"post[category]\"><option value=\"one\">one</option>\n<option value=\"two\">two</option></select>",
+      select("post", "category", CustomEnumerable.new)
+    )
+  end
+
   def test_collection_select
     @post = Post.new
     @post.author_name = "Babe"