Non-integer taxon keys in download request

This probably isn't what the user intended: https://www.gbif.org/occurrence/download/0015956-250827131500795

```
                {
                  "type": "equals",
                  "key": "TAXON_KEY",
                  "value": "5785887.0",
                  "matchCase": false
                }
````

(Created with PyGBIF.)