DbxDownloader.close() throws java.lang.IllegalStateException: Unbalanced enter/exit

Hi, I'm trying to cancel a download. My way to cancel is this.

```
public void cancelDownload(){
if (m_dbxDownloader != null) {
dbxDownloader.close();
dbxDownloader = null;
}
}
```
When I call dbxDownloader.close(); is thworing an exception. The exception is this

```
2019-02-22 09:53:14.576 26079-26412/com.toshiba.dse.ebridge.acme.lite E/ExceptionHandler: java.lang.IllegalStateException: Unbalanced enter/exit
at com.android.okhttp.okio.AsyncTimeout.enter(AsyncTimeout.java:62)
at com.android.okhttp.okio.AsyncTimeout$2.read(AsyncTimeout.java:209)
at com.android.okhttp.okio.RealBufferedSource.read(RealBufferedSource.java:60)
at com.android.okhttp.internal.http.HttpConnection$FixedLengthSource.read(HttpConnection.java:466)
at com.android.okhttp.internal.Util.skipAll(Util.java:164)
at com.android.okhttp.internal.Util.discard(Util.java:146)
at com.android.okhttp.internal.http.HttpConnection$FixedLengthSource.close(HttpConnection.java:483)
at com.android.okhttp.okio.RealBufferedSource.close(RealBufferedSource.java:421)
at com.android.okhttp.okio.RealBufferedSource$1.close(RealBufferedSource.java:409)
at com.dropbox.core.util.IOUtil.closeQuietly(IOUtil.java:149)
at com.dropbox.core.DbxDownloader.close(DbxDownloader.java:131)
```

Is this a problem with SDK?

Start at DbxDownloader.close() in DbxDownloader.java:131 and reproduce the cancelDownload sequence from the report. Trace the IOUtil.closeQuietly and Android OkHttp source-close frames to determine why cancellation causes “Unbalanced enter/exit.” Done means closing an active downloader no longer throws this exception, with coverage for the cancellation sequence.

Written by the indexing model from the issue text.