Fix week_field generating invalid week numbers (according to W3 and Browser)

[1st8]

Hey guys,

the week_field form helper currently uses %W when converting the input value. This results in week numbers being zero-based, which is invalid according to the W3 spec (http://www.w3.org/TR/html-markup/datatypes.html#form.data.week) and the browser in turn selects the week before the intended week.

The first commit adds a spec to illustrate the correct behavior, not receiving a value of 2015-W00 but expecting the correct 2015-W01.
The second commit fixes this spec by replacing %W with %V, differences according to the ruby doc:

Week number:
  %W - Week number of the year. The week starts with Monday. (00..53)

ISO 8601 week-based year and week number:
  %V - Week number of the week-based year (01..53)

Please let me know if I should fix/squash/append anything, since this is my first rails contribution 🎉

Cheers
Christoph

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @matthewd (or someone else) soon.

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[pixeltrix]

@1st8 nice catch! If you could add a CHANGELOG entry and squash the commits that'd be great.

[1st8]

Thanks for your feedback, it's done.

[pixeltrix]

@1st8 sorry, for the delay in merging - I just tweaked the commit message and changelog entry and then merged it in 320b79f. I've also backported it to 4-1-stable (4696e0d) and 4-2-stable (8f6ee25).

[pixeltrix]

@1st8 sorry, forgot to say thanks for you fix 😄

[1st8]

Very nice, thank you. I am happy to contribute! 🍻