RasterManager.add() fails to add image services.

Workaround

It is possible to resolve this issue by manually changing the source code, however only follow this instruction if it's required.

1. Make a backup of the _layer.py file referenced in the runtime error (e.g. /opt/anaconda3/envs/gdal/lib/python3.6/site-packages/arcgis/raster/_layer.py).
2. Open the original _layer.py file to perform edits.
3. Locate the _add_rasters function.
4. Modify the line that reads "service_url = [service_url]" to "service_url = service_url"
5. Save changes (if using a Jupyter Notebook also restart the kernel).

After making the above changes the function works as expected with a service URL.

Steps to Reproduce