Setting location_geometry to geoshape causes weird formatting

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Steps to reproduce the error

Rather than offering a choice of location_geoshape or location_geotrace, set location_geometry to geoshape. Fill out form, and submit. See regression in location geometry formatting.

Actual behavior

### Expected behavior

Completed polygon.

This is caused by the upgrade to allow selection of geotrace or geoshape. If there's isn't a selection, location_geometry assumes you're using geotrace.

https://github.com/Cadasta/cadasta-platform/blob/master/cadasta/xforms/mixins/model_helper.py#L76-L78