Upgrade to HTMLUnit 2.20

Closes gh-5599

spring-boot-dependencies/pom.xml

@@ -84,7 +84,7 @@
 		<hikaricp-java6.version>2.3.13</hikaricp-java6.version>
 		<hornetq.version>2.4.7.Final</hornetq.version>
 		<hsqldb.version>2.3.3</hsqldb.version>
-		<htmlunit.version>2.19</htmlunit.version>
+		<htmlunit.version>2.20</htmlunit.version>
 		<httpasyncclient.version>4.1.1</httpasyncclient.version>
 		<httpclient.version>4.5.2</httpclient.version>
 		<httpcore.version>4.4.4</httpcore.version>