Merge pull request #504 from Bhavik-P/checkKey

Check if addAndroidDownloads map has the notification key before consuming

android/src/main/java/com/RNFetchBlob/RNFetchBlobReq.java

             if (options.addAndroidDownloads.getBoolean("useDownloadManager")) {
                 Uri uri = Uri.parse(url);
                 DownloadManager.Request req = new DownloadManager.Request(uri);
-                if(options.addAndroidDownloads.getBoolean("notification")) {
+                if(options.addAndroidDownloads.hasKey("notification") && options.addAndroidDownloads.getBoolean("notification")) {
                     req.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
                 } else {
                     req.setNotificationVisibility(DownloadManager.Request.VISIBILITY_HIDDEN);