Add support for iso8601 YYYY-MM format

[jonwolski]

The iso 8601 spec allows for omission of the 'day' part of a date
representation. However, because it also allows omission of the month in
the case of YYYY-DDD (where DDD represents the day-of-year), there
is some abiguity.

If there are only two components, a four-digit year,
followed by a dash (-) followed by another series of digits, the
disambiguation is as follows:

• If there are two digits in the second component, that component
  represents the month.
• If there are three digits in the second component, that component
  represents the day-of-year.

The existing implementation has affordances for many things that are not
consistent with iso 8601. Rather than make a large,
backwards-incompatible change, I thought it best to fix this one edge
case.

https://bugs.ruby-lang.org/issues/12833

[sbussard]

It looks like there was a network error in AppVeyor when attempting to download zlib 1.2.8 from sourceforge. Someone with the correct permissions probably needs to run AppVeyor again, since the failure appears to be non-deterministic.

appveyor DownloadFile http://downloads.sourceforge.net/project/libpng/zlib/1.2.8/zlib128.zip
Error downloading remote file: One or more errors occurred.
Inner Exception: Unable to connect to the remote server
Command exited with code 2

[christophermancini]

👍 I am always for stricter adherence to standards. Tests pass on Travis and the code looks good to me.

[jonwolski]

This might be frowned upon, but a rebase to latest trunk and push -f triggered a successful (re-) build.

[jonwolski]

FWIW, https://github.com/arnau/ISO8601/blob/master/docs/date-time.md provides what I need. It would be slightly better if that gem allowed for monkey-patching the core date module to 'repair' it.

[jonwolski]

@nobu was this closed just to tidy up, or will this patch actually be accepted? Nevermind, I saw it on https://bugs.ruby-lang.org/issues/12833. Sorry to '@' you