Update to htmlunit:2.31

Fixes: gh-5433

gradle/dependency-management.gradle

@@ -89,7 +89,7 @@ dependencyManagement {
 		dependency 'net.sf.ehcache:ehcache:2.10.4'
 		dependency 'net.sourceforge.cssparser:cssparser:0.9.24'
 		dependency 'net.sourceforge.htmlunit:htmlunit-core-js:2.28'
-		dependency 'net.sourceforge.htmlunit:htmlunit:2.30'
+		dependency 'net.sourceforge.htmlunit:htmlunit:2.31'
 		dependency 'net.sourceforge.htmlunit:neko-htmlunit:2.28'
 		dependency 'net.sourceforge.nekohtml:nekohtml:1.9.22'
 		dependency 'nz.net.ultraq.thymeleaf:thymeleaf-expression-processor:1.1.3'