Allow Time.zone.at to receive a second argument

For parity with Ruby's Time::at

activesupport/lib/active_support/values/time_zone.rb

@@ -354,8 +354,13 @@ module ActiveSupport
     #   Time.zone = 'Hawaii'        # => "Hawaii"
     #   Time.utc(2000).to_f         # => 946684800.0
     #   Time.zone.at(946684800.0)   # => Fri, 31 Dec 1999 14:00:00 HST -10:00
-    def at(secs)
-      Time.at(secs).utc.in_time_zone(self)
+    #
+    # A second argument can be supplied to specify sub-second precision.
+    #
+    #   Time.zone = 'Hawaii'                # => "Hawaii"
+    #   Time.at(946684800, 123456.789).nsec # => 123456789
+    def at(*args)
+      Time.at(*args).utc.in_time_zone(self)
     end
 
     # Method for creating new ActiveSupport::TimeWithZone instance in time zone

activesupport/test/time_zone_test.rb

@@ -225,6 +225,16 @@ class TimeZoneTest < ActiveSupport::TestCase
     assert_equal secs, twz.to_f
   end
 
+  def test_at_with_microseconds
+    zone = ActiveSupport::TimeZone["Eastern Time (US & Canada)"]
+    secs = 946684800.0
+    microsecs = 123456.789
+    twz = zone.at(secs, microsecs)
+    assert_equal zone, twz.time_zone
+    assert_equal secs, twz.to_i
+    assert_equal 123456789, twz.nsec
+  end
+
   def test_iso8601
     zone = ActiveSupport::TimeZone["Eastern Time (US & Canada)"]
     twz = zone.iso8601("1999-12-31T19:00:00")