Preserve time zone offset when deserializing times

[stomar]

Times with specified time zone offset are converted to local time, currently. I consider this a bug, because information that is present in the YAML gets lost needlessly.

I think the upcoming major version release would be a good occasion to fix this.

Current behavior:

require "psych"

ENV["TZ"] = "EST"

time = Time.new(2017, 4, 13, 12, 0, 0, "+02:00")
yaml = Psych.dump(time)
restored = Psych::load yaml

restored == time  # => true, it's the same point in time

yaml      # => "--- 2017-04-13 12:00:00.000000000 +02:00\n...\n"
time      # => 2017-04-13 12:00:00 +0200
restored  # => 2017-04-13 05:00:00 -0500 (!)

New behavior:

# ...
yaml      # => "--- 2017-04-13 12:00:00.000000000 +02:00\n...\n"
time      # => 2017-04-13 12:00:00 +0200
restored  # => 2017-04-13 12:00:00 +0200

Note that I am not at all familiar with the psych codebase, so feel free to give feedback if something is missing.

Update: The first commit adds "bonus" test cases that do not directly concern the proposed change, but seemed reasonable.

Update: DateTime will change similarly (without need for special treatment; it seems to rely on Time). I amended the commits to include a test case that verifies this behavior.

[stomar]

@tenderlove @hsbt Any thoughts on this?

[tenderlove]

@stomar thanks for the bump! Yes, I think this is good. I'll merge! 😄

[stomar]

Wow, that was fast... thanks!

[stomar]

@hsbt Will this be included in the preview of 2.5?

[hsbt]

@stomar Yes, I will merge current master into Ruby 2.5 and release pysch-3.0.0.beta2.