Upgrade to Httpclient 4.5.9

Closes gh-17666

Author: snicoll

spring-boot-dependencies/pom.xml

@@ -93,7 +93,7 @@
 		<hsqldb.version>2.3.6</hsqldb.version>
 		<htmlunit.version>2.21</htmlunit.version>
 		<httpasyncclient.version>4.1.4</httpasyncclient.version>
-		<httpclient.version>4.5.8</httpclient.version>
+		<httpclient.version>4.5.9</httpclient.version>
 		<httpcore.version>4.4.11</httpcore.version>
 		<infinispan.version>8.2.11.Final</infinispan.version>
 		<jackson.version>2.8.11.20181123</jackson.version>