defualt http manager construct

Author: longbai

library/src/main/java/com/qiniu/android/http/HttpManager.java

@@ -20,7 +20,6 @@ public final class HttpManager {
     private static final String userAgent = getUserAgent();
     private AsyncHttpClient client;
 
-
     public HttpManager(Proxy proxy) {
         client = new AsyncHttpClient();
         client.setConnectTimeout(Config.CONNECT_TIMEOUT);
@@ -32,6 +31,10 @@ public HttpManager(Proxy proxy) {
         }
     }
 
+    public HttpManager() {
+        this(null);
+    }
+
     private static String genId() {
         Random r = new Random();
         return System.currentTimeMillis() + "" + r.nextInt(999);