Year setter should keep time when DST changes

[mattjohnsonpint]

Consider:

moment.tz("2016-03-28T00:00:00","Europe/London").year(2015).format()  //  "2015-03-27T23:00:00+00:00"

It should have been "2015-03-28T00:00:00+00:00"

While this only manifests when using moment-timezone, the problem is in moment itself. The keepTime flag of the year unit should be set to true, as it is for months, days, and hours.

Fixed this, added some unit tests, and added similar tests around other units and related scenarios.

[maggiepint]

Code looks fine, but did you want a commit with no tests?
I have a weird thing about always wanting to see test coverage with every commit when reading history, but maybe that's just me.

[mattjohnsonpint]

Re-based to one commit so tests are associated in the commit history. Thanks.

[ichernev]

@mj1856 hm, weird why it was created broken. Thanks for the fix!

[ichernev]

Merged in e52fd3a