Upgrade to Xmlunit2 2.6.4

Closes gh-20657

Author: snicoll

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

@@ -214,7 +214,7 @@
 		<webjars-hal-browser.version>3325375</webjars-hal-browser.version>
 		<webjars-locator-core.version>0.41</webjars-locator-core.version>
 		<wsdl4j.version>1.6.3</wsdl4j.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>