Fix TimeWithZone#to_time to return self

A previous commit (7872cc9) changed it to return utc to simplify the Time#<==>
override. An alternative method acheives the same result while
preserving original behaviour.

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

@@ -379,9 +379,10 @@ def minus_with_coercion(other)
   # Layers additional behavior on Time#<=> so that DateTime and ActiveSupport::TimeWithZone instances
   # can be chronologically compared with a Time
   def compare_with_coercion(other)
+    other = other.comparable_time if other.respond_to?(:comparable_time)
     # we're avoiding Time#to_datetime cause it's expensive
     if other.is_a?(Time)
-      compare_without_coercion(other.to_time)
+      compare_without_coercion(other)
     else
       to_datetime <=> other
     end

activesupport/lib/active_support/time_with_zone.rb

@@ -279,7 +279,7 @@ def to_i
 
     # A TimeWithZone acts like a Time, so just return +self+.
     def to_time
-      utc
+      self
     end
 
     def to_datetime

activesupport/test/core_ext/time_with_zone_test.rb

@@ -319,7 +319,7 @@ def test_to_i_with_wrapped_datetime
   end
 
   def test_to_time
-    assert_equal @twz, @twz.to_time
+    assert @twz.equal?(@twz.to_time)
   end
 
   def test_to_date