Make ActiveSupport::TimeWithZone#xmlschema consistent

Both Time#xmlschema and DateTime#xmlschema can accept nil values for the
fraction_digits parameter. This commit makes this so for TimeWithZone
values as well.

activesupport/lib/active_support/time_with_zone.rb

@@ -132,8 +132,8 @@ def inspect
     end
 
     def xmlschema(fraction_digits = 0)
-      fraction = if fraction_digits > 0
-        (".%06i" % time.usec)[0, fraction_digits + 1]
+      fraction = if fraction_digits.to_i > 0
+        (".%06i" % time.usec)[0, fraction_digits.to_i + 1]
       end
 
       "#{time.strftime("%Y-%m-%dT%H:%M:%S")}#{fraction}#{formatted_offset(true, 'Z')}"

activesupport/test/core_ext/time_with_zone_test.rb

@@ -111,6 +111,10 @@ def test_xmlschema_with_fractional_seconds_lower_than_hundred_thousand
     assert_equal "1999-12-31T19:00:00.001234-05:00", @twz.xmlschema(12)
   end
 
+  def test_xmlschema_with_nil_fractional_seconds
+    assert_equal "1999-12-31T19:00:00-05:00", @twz.xmlschema(nil)
+  end
+
   def test_to_yaml
     assert_match(/^--- 2000-01-01 00:00:00(\.0+)?\s*Z\n/, @twz.to_yaml)
   end