Disable GPX export for polygons

Hello,
It would be interesting to add the gpx format to the list in order to easily load the data into a GPS.

```
'gpx':{
        'contentType': 'application/gpx+xml',
        'filenameExt': 'gpx',
        'forceCRS': 'EPSG:4326',
        'ogrProvider': 'GPX',
        'ogrDatasourceOptions':None,
        'zip': False,
        'extToZip': []
    },
```
Thanks