Fix week_field returning invalid value

According to the W3 spec[1] the value should use a 1-based index
and not a 0-based index for the week number.

[1]: http://www.w3.org/TR/html-markup/datatypes.html#form.data.week

(cherry picked from commit 60dabb1)

Author: Christoph

actionview/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Generate `week_field` input values using a 1-based index and not a 0-based index
+    as per the W3 spec: http://www.w3.org/TR/html-markup/datatypes.html#form.data.week
+
+    *Christoph Geschwind*
+
+
 ## Rails 4.1.14.rc1 (October 30, 2015) ##
 
 *   No changes.

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

@@ -5,7 +5,7 @@ class WeekField < DatetimeField # :nodoc:
         private
 
           def format_date(value)
-            value.try(:strftime, "%Y-W%W")
+            value.try(:strftime, "%Y-W%V")
           end
       end
     end

actionview/test/template/form_helper_test.rb

@@ -912,7 +912,7 @@ def test_month_field_with_timewithzone_value
   end
 
   def test_week_field
-    expected = %{<input id="post_written_on" name="post[written_on]" type="week" value="2004-W24" />}
+    expected = %{<input id="post_written_on" name="post[written_on]" type="week" value="2004-W25" />}
     assert_dom_equal(expected, week_field("post", "written_on"))
   end
 
@@ -923,13 +923,13 @@ def test_week_field_with_nil_value
   end
 
   def test_week_field_with_datetime_value
-    expected = %{<input id="post_written_on" name="post[written_on]" type="week" value="2004-W24" />}
+    expected = %{<input id="post_written_on" name="post[written_on]" type="week" value="2004-W25" />}
     @post.written_on = DateTime.new(2004, 6, 15, 1, 2, 3)
     assert_dom_equal(expected, week_field("post", "written_on"))
   end
 
   def test_week_field_with_extra_attrs
-    expected = %{<input id="post_written_on" step="2" max="2010-W51" min="2000-W06" name="post[written_on]" type="week" value="2004-W24" />}
+    expected = %{<input id="post_written_on" step="2" max="2010-W51" min="2000-W06" name="post[written_on]" type="week" value="2004-W25" />}
     @post.written_on = DateTime.new(2004, 6, 15, 1, 2, 3)
     min_value = DateTime.new(2000, 2, 13)
     max_value = DateTime.new(2010, 12, 23)
@@ -939,13 +939,19 @@ def test_week_field_with_extra_attrs
 
   def test_week_field_with_timewithzone_value
     previous_time_zone, Time.zone = Time.zone, 'UTC'
-    expected = %{<input id="post_written_on" name="post[written_on]" type="week" value="2004-W24" />}
+    expected = %{<input id="post_written_on" name="post[written_on]" type="week" value="2004-W25" />}
     @post.written_on = Time.zone.parse('2004-06-15 15:30:45')
     assert_dom_equal(expected, week_field("post", "written_on"))
   ensure
     Time.zone = previous_time_zone
   end
 
+  def test_week_field_week_number_base
+    expected = %{<input id="post_written_on" name="post[written_on]" type="week" value="2015-W01" />}
+    @post.written_on = DateTime.new(2015, 1, 1, 1, 2, 3)
+    assert_dom_equal(expected, week_field("post", "written_on"))
+  end
+
   def test_url_field
     expected = %{<input id="user_homepage" name="user[homepage]" type="url" />}
     assert_dom_equal(expected, url_field("user", "homepage"))