Fix: timezone bug - rounding problem

Why this commit?

   TimeWithZone instances were unexpectedly being rounded up.

What changes were made and why?

   Internally, .to_f was being called on TimeWithZone instances.
This can lead to inaccuracies if Rationals are involved. Using .to_r
instead of .to_f will be more accurate, but it does come with a
slight computational cost.

activesupport/lib/active_support/core_ext/time/calculations.rb

@@ -47,7 +47,9 @@ def at_with_coercion(*args)
       # Time.at can be called with a time or numerical value
       time_or_number = args.first
 
-      if time_or_number.is_a?(ActiveSupport::TimeWithZone) || time_or_number.is_a?(DateTime)
+      if time_or_number.is_a?(ActiveSupport::TimeWithZone)
+        at_without_coercion(time_or_number.to_r).getlocal
+      elsif time_or_number.is_a?(DateTime)
         at_without_coercion(time_or_number.to_f).getlocal
       else
         at_without_coercion(time_or_number)

activesupport/test/core_ext/time_with_zone_test.rb

@@ -1162,6 +1162,15 @@ def test_use_zone_raises_on_invalid_timezone
     assert_equal ActiveSupport::TimeZone["Alaska"], Time.zone
   end
 
+  def test_end_of_period_timezone_equality_with_DateTime
+    Time.use_zone "UTC" do
+      without_time_zone = "2019-01-01 00:00:00Z".to_time.end_of_month
+      zoned = without_time_zone.in_time_zone
+
+      assert_equal(Time.zone.at(without_time_zone), Time.zone.at(zoned))
+    end
+  end
+
   def test_time_zone_getter_and_setter
     Time.zone = ActiveSupport::TimeZone["Alaska"]
     assert_equal ActiveSupport::TimeZone["Alaska"], Time.zone