Nano seconds fraction of time is copied properly in Time#advance

When day, month or year was passed, advance created a new time
ignoring previous nsec fraction. Now nsec is passed through
usec as a Rational number.

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

@@ -78,7 +78,7 @@ def change(options)
       options[:hour]  || hour,
       options[:min]   || (options[:hour] ? 0 : min),
       options[:sec]   || ((options[:hour] || options[:min]) ? 0 : sec),
-      options[:usec]  || ((options[:hour] || options[:min] || options[:sec]) ? 0 : usec)
+      options[:usec]  || ((options[:hour] || options[:min] || options[:sec]) ? 0 : Rational(nsec, 1000))
     )
   end
 

activesupport/test/core_ext/time_ext_test.rb

@@ -491,6 +491,11 @@ def test_utc_advance
     assert_equal Time.utc(2013,10,17,20,22,19), Time.utc(2005,2,28,15,15,10).advance(:years => 7, :months => 19, :weeks => 2, :days => 5, :hours => 5, :minutes => 7, :seconds => 9)
   end
 
+  def test_advance_with_nsec
+    t = Time.at(0, Rational(108635108, 1000))
+    assert_equal t, t.advance(:months => 0)
+  end
+
   def test_prev_week
     with_env_tz 'US/Eastern' do
       assert_equal Time.local(2005,2,21), Time.local(2005,3,1,15,15,10).prev_week