Implementation difference between geojson js vs geojson java

This issue is raised from PR #1075 .

• In the js implementation of geojson, the properties value of a Feature can be empty, but in the java implementation, an empty properties field is ignored:

https://github.com/mapbox/mapbox-java/blob/2130ea708c72ef8b98aeb02102554d7dc32606a0/services-geojson/src/main/java/com/mapbox/geojson/Feature.java#L280

• When converting js version of geojson to the java version, the precision of coordinates has been reduced, i.e. from 102.227783203125 to 102.2277832. This issue was introduced in #938 . It seems the precision is trimed due to performance consideration.

从 Feature.java 中链接的 properties 处理开始，并查看 PR #1075 和 issue #938 以了解行为变化。比较 Java 输出与 JavaScript GeoJSON 对空 properties 和坐标精度的行为；当两个差异都得到解决并由相关测试覆盖时，即视为完成。