fix for geoJSONFeature attributes rewrite by closure compiler

GeoJSONFeatures often used to tranfser features via network. When you compile geojsonformat.js with Closure Compiler (advanced optimizations enabled) the properties (like 'type', 'coordinates',...) get renamed. When you load a GeoJSONFeature via Network the property names in the code doesn't match the property names on the wire anymore. 

see https://developers.google.com/closure/compiler/docs/api-tutorial3#propnames for details.

> This solution is pretty simple. Whenever possible, use dot-syntax property names rather than quoted strings. Use quoted string property names only when you don't want Closure Compiler to rename a property at all.

And we don't want that Closure Compiler rename the properties.