Improve performance of DateTime#seconds_since_unix_epoch

Calculate the seconds since the UNIX epoch using the difference in
Julian day numbers from the epoch date. By reducing the Rational math
to just the offset component this gives a significant improvement.

Benchmark:

Calculating --------------------------------------------
       new     27733 i/100ms
   current     15031 i/100ms
       new     27737 i/100ms
   current     15549 i/100ms
--------------------------------------------------------
       new   548182.1 (±0.9%) i/s - 2745567 in 5.008943s
   current   216380.9 (±1.6%) i/s - 1082232 in 5.002781s
       new   510281.9 (±1.2%) i/s - 2551804 in 5.001525s
   current   219858.3 (±1.8%) i/s - 1103979 in 5.023039s

activesupport/lib/active_support/core_ext/date_time/conversions.rb

@@ -80,8 +80,11 @@ def to_i
 
   private
 
+  def offset_in_seconds
+    (offset * 86400).to_i
+  end
+
   def seconds_since_unix_epoch
-    seconds_per_day = 86_400
-    (self - ::DateTime.civil(1970)) * seconds_per_day
+    (jd - 2440588) * 86400 - offset_in_seconds + seconds_since_midnight
   end
 end

activesupport/test/core_ext/date_time_ext_test.rb

@@ -427,6 +427,7 @@ def test_to_f
 
   def test_to_i
     assert_equal 946684800, DateTime.civil(2000).to_i
+    assert_equal 946684800, DateTime.civil(1999,12,31,19,0,0,Rational(-5,24)).to_i
   end
 
   protected