fix bug with numeric properties in custom map data

[k-yle]

If you upload a geojson file under "Custom Map Data" with properties that are numbers, the tag editor gets confused:

This PR fixes that bug. It can be tested with this file:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": { "a": 1 },
      "geometry": {
        "type": "LineString",
        "coordinates": [
          [174.70733642578122, -36.83456911447563],
          [174.90234375, -36.83237080155681]
        ]
      }
    },
    {
      "type": "Feature",
      "properties": { "a": 2 },
      "geometry": {
        "type": "LineString",
        "coordinates": [
          [174.89273071289062, -36.80928470205938],
          [174.94491577148438, -36.890604744644705]
        ]
      }
    }
  ]
}

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[tyrasd]

I think if we address this, we should address all data types that might potentially be included in a GeoJSON file. Also the raw tag editor module was not the right place to fix this. I've pushed 3373f1b to your branch, which should address both of these issues. Let me know if that is fine for you.

[k-yle]

thanks @tyrasd !