change test for nanosecond accurate timestamps

ruby/tests/well_known_types_test.rb

@@ -13,15 +13,18 @@ class TestWellKnownTypes < Test::Unit::TestCase
     assert_equal Time.at(12345), ts.to_time
     assert_equal 12345, ts.to_i
 
+    # millisecond accuracy
     time = Time.at(123456, 654321)
     ts.from_time(time)
     assert_equal 123456, ts.seconds
     assert_equal 654321000, ts.nanos
     assert_equal time, ts.to_time
 
-    time = Time.now
+    # nanosecond accuracy
+    time = Time.at(123456, Rational(654321321, 1000))
     ts.from_time(time)
-    assert_equal time.to_f, ts.to_time.to_f
+    assert_equal 654321321, ts.nanos
+    assert_equal time, ts.to_time
   end
 
   def test_duration