Upgrade to Xmlunit2 2.6.4

Closes gh-21166

Author: snicoll

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -198,7 +198,7 @@
 		<woodstox.version>5.0.3</woodstox.version>
 		<wsdl4j.version>1.6.3</wsdl4j.version>
 		<xml-apis.version>1.4.01</xml-apis.version>
-		<xmlunit2.version>2.6.3</xmlunit2.version>
+		<xmlunit2.version>2.6.4</xmlunit2.version>
 		<!-- Plugin versions -->
 		<build-helper-maven-plugin.version>3.0.0</build-helper-maven-plugin.version>
 		<exec-maven-plugin.version>1.6.0</exec-maven-plugin.version>