found error case using intersection operation

Sorry to mention this issue to you, but I found a test case in `w8r/Martinez` repo `self_intersection.geojson` [link](https://github.com/w8r/martinez/blob/3b85d7a6ddf7de85b5ef61a929f57ccf2331eb05/test/fixtures/self_intersecting.geojson) which outputs the wrong result, plz check this if you have time, appreciate it!