Fix ActiveSupport::TimeWithZone#in

[tbalthazar]

Summary

ActiveSupport::TimeWithZone#in is now an alias for ActiveSupport::TimeWithZone#+ instead of being sent to a Time instance (via method_missing). It no longer returns wrong data when used on a TimeWithZone near DST.

Example wrong data that was returned before the fix:

Time.zone = ActiveSupport::TimeZone.new("America/New_York")
t = Time.zone.local(2016,11,6,1) # Sun, 06 Nov 2016 01:00:00 EDT -04:00
t + 1.hour   # ok => Sun, 06 Nov 2016 01:00:00 EST -05:00
t.in(1.hour) # ko => Sun, 06 Nov 2016 02:00:00 EST -05:00

It fixes #26580.

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[pixeltrix]

@tbalthazar code is 👍 but can I just ask for a couple of things:

1. Can you change the CHANGELOG entry to this:

   *   Fix `ActiveSupport::TimeWithZone#in` across DST boundaries.
   
   Previously calls to `in` were being sent to the non-DST aware
   method `Time#since` via `method_missing`. It is now aliased to
   the DST aware `ActiveSupport::TimeWithZone#+` which handles
   transitions across DST boundaries, e.g:
   
       Time.zone = "US/Eastern"
   
       t = Time.zone.local(2016,11,6,1)
       # => Sun, 06 Nov 2016 01:00:00 EDT -05:00 
   
       t.in(1.hour)
       # => Sun, 06 Nov 2016 01:00:00 EST -05:00 
   
   Fixes #26580.
   
   *Thomas Balthazar*

2. Can you rebase and squash the merge commits

Thanks ❤️

[tbalthazar]

@pixeltrix Great suggestion, your changelog entry is better written, thanks!
Looks like the build failed this time, but I'm pretty sure it's not related to this commit.
Could you please relaunch the Travis checks? (Or is there any way I can do that myself?)
Thanks!

[pixeltrix]

@tbalthazar restarted the Travis jobs 👍

[pixeltrix]

Backported to 5-0-stable in 0a7176c and 4-2-stable in a6edbeb.

Slightly complicated by the fact that up until #18063 since was a method on AS::TWZ and given the (slight) breaking change by that PR it made sense to adjust the alias to since instead of +.