"Unable to parse WKT" when changing the geofield's widget

Steps to reproduce:

1. Create a content type with some text field and Geospatial data field with "Latitude/Longtitude" widget.

2. Try after this to change the widget type to "Geocoded value from another field".

3. When trying to save the field settings, an error is shown: "Unable to parse WKT." and "The default value is invalid.".

The problem is in the validate code in geo_field.module:

// Convert to $wkt and validate it.
$wkt = $item['geo'];     
if ($gis_input) {
  $wkt = geo_value($wkt, 'wkt', $gis_input);
}

When changing the widget from "Latitude/Longtitude" to "Geocoded value from another field" widget, the $gis_input is empty and the geocoding doesn't take place.
On the other hand the $wkt in that case is an array (Array ( [lat] => [lon] => ) ) so it must be geocoded.
And since the $wkt is sent without been geocoded, the wkt_validate function returns an error.

One solution to this problem is to check if the $wkt is an array and to set the $gis_input if empty.

This is done by the patch I have attached. The patch is created against HEAD.