Fix for java.lang.ClassCastException in BinaryHttpResponseHandler

I've got loads of `ClassCastException` crash reports and this commit might fix the bug.
I'm not yet sure of why this bug exists in the first place.
Please confirm this and let me know if I'm doing something wrong here.
Thanks

```java
java.lang.ClassCastException: java.lang.String cannot be cast to byte[]
  at com.loopj.android.http.BinaryHttpResponseHandler.handleMessage(BinaryHttpResponseHandler.java:145)
  at com.loopj.android.http.AsyncHttpResponseHandler$1.handleMessage(AsyncHttpResponseHandler.java:84)
```