HTTPCLIENT-1123: DNS resolver null parameter check

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1181737 13f79535-47bb-0310-9956-ffa450edef68

httpclient/src/main/java/org/apache/http/impl/conn/PoolingClientConnectionManager.java

@@ -106,6 +106,9 @@ public class PoolingClientConnectionManager implements ClientConnectionManager,
         if (schemeRegistry == null) {
             throw new IllegalArgumentException("Scheme registry may not be null");
         }
+        if (dnsResolver == null) {
+            throw new IllegalArgumentException("DNS resolver may not be null");
+        }
         this.schemeRegistry = schemeRegistry;
         this.dnsResolver  = dnsResolver;
         this.operator = createConnectionOperator(schemeRegistry);