[2.3] Always convert Time and DateTime objects to local before converting to string

[haileys]

This breaks a whole bunch of Rails tests, which is to be expected because there was previously no timezone handling here at all.

cc @github/timezones @github/rails3 github/github#27189

[rsanheim]

If we go this route, how should TimeWithZone behave? Right now, to_s(:db) for any time with zone will always output UTC:

    # <tt>:db</tt> format outputs time in UTC; all others output time in local.
    # Uses TimeWithZone's +strftime+, so <tt>%Z</tt> and <tt>%z</tt> work correctly.
    def to_s(format = :default)
      return utc.to_s(format) if format == :db
      if formatter = ::Time::DATE_FORMATS[format]
        formatter.respond_to?(:call) ? formatter.call(self).to_s : strftime(formatter)
      else
        "#{time.strftime("%Y-%m-%d %H:%M:%S")} #{formatted_offset(false, 'UTC')}" # mimicking Ruby 1.9 Time#to_s format
      end
    end
    alias_method :to_formatted_s, :to_s

[haileys]

@rsanheim Yeah, that's tricky. It should return local for us. I believe Rails 3 makes this configurable.