Update GeoJSON.js

I was only able to use the Point. When I tried to use the Polygon, i got the following stack trace:

..\data\node_modules\mongoose\lib\schema\array.js:58
    this.caster = new caster(null, castOptions);
                  ^
TypeError: string is not a function at new SchemaArray 
..\data\node_modules\mongoose\lib\schema\array.js:58:19)
    at Function.Schema.interpretAsType 

By using the native types, I am able to avoid the ambiguity of string casting. And without including the mongoose schema for Mixed type and using object, I am able to still insert anything I want.