modify dns cache logic

Author: YangSen-qn

library/src/androidTest/java/com/qiniu/android/AsynTest.java

@@ -70,7 +70,7 @@ public void run() {
                         @Override
                         public void run() {
 
-                            if (Thread.currentThread().getName().equals("main")){
+                            if (Thread.currentThread() == Looper.getMainLooper().getThread()){
                                 testParam.successCount += 1;
                             }
 
@@ -112,7 +112,7 @@ public void testAsyncBg(){
                 @Override
                 public void run() {
                     String threadName = Thread.currentThread().getName();
-                    if (!threadName.equals("main")){
+                    if (Thread.currentThread() != Looper.getMainLooper().getThread()){
                         synchronized (this){
                             testParam.successCount += 1;
                         }

library/src/main/java/com/qiniu/android/http/dns/DnsCacheFile.java

@@ -29,6 +29,10 @@ public DnsCacheFile(String directory) throws IOException {
                 throw new IOException("mkdir failed");
             }
         }
+
+        if (!f.isDirectory()) {
+            throw new IOException("does not mkdir");
+        }
     }
 
     /**