xmlformatter tests are non-determanistic

gistfile1.cs

Test 'Nancy.Formatters.Tests.XmlFormatterExtensionsFixtures.Should_return_a_valid_model_in_xml_format' failed: Assert.Equal() Failure
Position: First difference is at position 39
Expected: <?xml version="1.0"?>
          <Person xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
            <FirstName>Andy</FirstName>
            <LastName>Pike</LastName>
          </Person>
Actual:   <?xml version="1.0"?>
          <Person xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
            <FirstName>Andy</FirstName>
            <LastName>Pike</LastName>
          </Person>
	ShouldExtensions.cs(58,0): at Nancy.Tests.ShouldAssertExtensions.ShouldEqual(Object actual, Object expected)
	ShouldExtensions.cs(128,0): at Nancy.Tests.ShouldAssertExtensions.ShouldEqual(MemoryStream stream, String expected)
	XmlFormatterExtensionsFixtures.cs(41,0): at Nancy.Formatters.Tests.XmlFormatterExtensionsFixtures.Should_return_a_valid_model_in_xml_format()

Test 'Nancy.Formatters.Tests.XmlFormatterExtensionsFixtures.Should_return_a_null_in_xml_format' failed: Assert.Equal() Failure
Position: First difference is at position 39
Expected: <?xml version="1.0"?>
          <Person xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true" />
Actual:   <?xml version="1.0"?>
          <Person xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xsi:nil="true" />
	ShouldExtensions.cs(58,0): at Nancy.Tests.ShouldAssertExtensions.ShouldEqual(Object actual, Object expected)
	ShouldExtensions.cs(128,0): at Nancy.Tests.ShouldAssertExtensions.ShouldEqual(MemoryStream stream, String expected)
	XmlFormatterExtensionsFixtures.cs(52,0): at Nancy.Formatters.Tests.XmlFormatterExtensionsFixtures.Should_return_a_null_in_xml_format()