Clear connection pool for OkHttpClient

Closes gh-23628

spring-web/src/main/java/org/springframework/http/client/OkHttp3ClientHttpRequestFactory.java

 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -121,6 +121,7 @@ public class OkHttp3ClientHttpRequestFactory
 				cache.close();
 			}
 			this.client.dispatcher().executorService().shutdown();
+			this.client.connectionPool().evictAll();
 		}
 	}