fix time_tag helper and update tests to not rely on dynamically generated values

Before `time_tag Date.today` will return `<time
  datetime="2013-02-14T00:00:00+00:00">February 14, 2013</time>`.
  This commit fixes time_tag helper to use RFC-3339 full-date format
  (Y-m-d) in datetime attribute.

actionpack/lib/action_view/helpers/date_helper.rb

@@ -651,7 +651,7 @@ module ActionView
         options  = args.extract_options!
         format   = options.delete(:format) || :long
         content  = args.first || I18n.l(date_or_time, :format => format)
-        datetime = date_or_time.acts_like?(:time) ? date_or_time.xmlschema : date_or_time.rfc3339
+        datetime = date_or_time.acts_like?(:time) ? date_or_time.xmlschema : date_or_time.iso8601
 
         content_tag(:time, content, options.reverse_merge(:datetime => datetime), &block)
       end

actionpack/test/template/date_helper_test.rb

@@ -3160,14 +3160,14 @@ class DateHelperTest < ActionView::TestCase
   end
 
   def test_time_tag_with_date
-    date = Date.today
-    expected = "<time datetime=\"#{date.rfc3339}\">#{I18n.l(date, :format => :long)}</time>"
+    date = Date.new(2013, 2, 20)
+    expected = '<time datetime="2013-02-20">February 20, 2013</time>'
     assert_equal expected, time_tag(date)
   end
 
   def test_time_tag_with_time
-    time = Time.now
-    expected = "<time datetime=\"#{time.xmlschema}\">#{I18n.l(time, :format => :long)}</time>"
+    time = Time.new(2013, 2, 20, 0, 0, 0, '+00:00')
+    expected = '<time datetime="2013-02-20T00:00:00+00:00">February 20, 2013 00:00</time>'
     assert_equal expected, time_tag(time)
   end
 
@@ -3184,8 +3184,8 @@ class DateHelperTest < ActionView::TestCase
   end
 
   def test_time_tag_with_different_format
-    time = Time.now
-    expected = "<time datetime=\"#{time.xmlschema}\">#{I18n.l(time, :format => :short)}</time>"
+    time = Time.new(2013, 2, 20, 0, 0, 0, '+00:00')
+    expected = '<time datetime="2013-02-20T00:00:00+00:00">20 Feb 00:00</time>'
     assert_equal expected, time_tag(time, :format => :short)
   end