Fixing java.util.concurrent.RejectedExecutionException

Fixing a crash when a download task is rejected from the tread pool.

java.util.concurrent.RejectedExecutionException: Task android.os.AsyncTask$3@42dcacb0 rejected from java.util.concurrent.ThreadPoolExecutor@420cbee0[Running, pool size = 9, active threads = 9, queued tasks = 128, completed tasks = 55]
       at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2011)
       at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:793)
       at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1339)
       at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:590)
       at com.koushikdutta.urlimageviewhelper.UrlImageViewHelper.executeTaskHoneycomb(SourceFile:805)
       at com.koushikdutta.urlimageviewhelper.UrlImageViewHelper.executeTask(SourceFile:799)
       at com.koushikdutta.urlimageviewhelper.HttpUrlDownloader.download(SourceFile:76)
       at com.koushikdutta.urlimageviewhelper.UrlImageViewHelper.setUrlDrawable(SourceFile:669)
       at com.koushikdutta.urlimageviewhelper.UrlImageViewHelper.setUrlDrawable(SourceFile:157)