[GEO] GeoShapeFieldMapper incorrectly acknowledges mapping changes

[nknize]

GeoShapeFieldMapper does not overload merge(), so the following request produces
acknowledged: "true" from the AbstractFieldMapper

# update mapping after inserting a shape
PUT geo-shapes/doc/_mapping
{
  "doc": {
    "properties": {
        "geometry": {
          "type": "geo_shape",
          "tree": "quadtree",
          "tree_levels": 26,
          "distance_error_pct": 0
        }
    }
  }
}

This is misleading since changing field parameters has zero impact on the GeoShapeFieldMapper itself leading to a user thinking they're indexing at a higher precision when no change has occurred. This is bad news for query expectations.

This issue will be corrected by overloading merge to add conflicts if certain parameters are changed.