Properly close test HttpServer, close AsyncHttpClient#1434

Author: slandelle

client/src/test/java/org/asynchttpclient/testserver/HttpServer.java

@@ -119,7 +119,7 @@ public void reset() {
 
     @Override
     public void close() throws IOException {
-        if (server == null) {
+        if (server != null) {
             try {
                 server.stop();
             } catch (Exception e) {