`DateTime#to_f` now preserves fractional seconds.

Previously this method always returns `.0` in the fractional part. This commit
changes it to preserve the fractional seconds instead.

Fixes #15994.

activesupport/CHANGELOG.md

+*   `DateTime#to_f` now preserves the fractional seconds instead of always
+    rounding to `.0`.
+
+    Fixes #15994.
+
+    *John Paul Ashenfelter*
+
 *   Add `Hash#transform_values` to simplify a common pattern where the values of a
     hash must change, but the keys are left the same.
 

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

@@ -71,9 +71,9 @@ def self.civil_from_format(utc_or_local, year, month=1, day=1, hour=0, min=0, se
     civil(year, month, day, hour, min, sec, offset)
   end
 
-  # Converts +self+ to a floating-point number of seconds since the Unix epoch.
+  # Converts +self+ to a floating-point number of seconds, including fractional microseconds, since the Unix epoch.
   def to_f
-    seconds_since_unix_epoch.to_f
+    seconds_since_unix_epoch.to_f + sec_fraction
   end
 
   # Converts +self+ to an integer number of seconds since the Unix epoch.

activesupport/test/core_ext/date_time_ext_test.rb

@@ -338,6 +338,7 @@ def test_compare_with_time_with_zone
   def test_to_f
     assert_equal 946684800.0, DateTime.civil(2000).to_f
     assert_equal 946684800.0, DateTime.civil(1999,12,31,19,0,0,Rational(-5,24)).to_f
+    assert_equal 946684800.5, DateTime.civil(1999,12,31,19,0,0.5,Rational(-5,24)).to_f
   end
 
   def test_to_i