Merge pull request #1270 from lingochamp/fix/cast_exception

fix: may throw cast exception in FileDownloadThreadPool

Author: Jacksgong

library/src/main/java/com/liulishuo/filedownloader/services/FileDownloadThreadPool.java

@@ -121,7 +121,7 @@ private synchronized void filterOutNoExist() {
         runnablePool = correctedRunnablePool;
     }
 
-    public boolean isInThreadPool(final int downloadId) {
+    public synchronized boolean isInThreadPool(final int downloadId) {
         final DownloadLaunchRunnable runnable = runnablePool.get(downloadId);
         return runnable != null && runnable.isAlive();
     }