Faces get flipped on conversion

Hi,

We have had issue with some of our OBJs, and after taking the time to look into this a lot more deeply, I'm at a loss to what is happening.

Here is a very simple [example OBJ](https://github.com/AnalyticalGraphicsInc/obj2gltf/files/3672586/test_original.obj.zip)

It looks like this

When I convert it with the command `obj2gltf.js -i test_original.obj -o test_original.gltf`, I get [a glTF](https://github.com/AnalyticalGraphicsInc/obj2gltf/files/3672648/test_original.gltf.zip) that looks like this

As you can see, the faces with normals along the X axis get flipped.

Where things get even weirder, is that my original OBJ has the following vertex normals:
```
vn 0 0 1
vn 1 0 -0
vn 1 0 0
vn -1 0 -0
```

If I go and replace it with (manually flipping the X axis)
```
vn 0 0 1
vn -1 0 -0
vn -1 0 0
vn 1 0 -0
```
I get [this OBJ](https://github.com/AnalyticalGraphicsInc/obj2gltf/files/3672628/test_original_x_flipped.obj.zip), and visually, it is the same as my original (which I don't really get).

In any case, converting it to [glTF](https://github.com/AnalyticalGraphicsInc/obj2gltf/files/3672647/test_original_x_flipped.gltf.zip) gives me the expected visual result now

As a bit of context, my original OBJ is in fact a triangulation (by importing/exporting to Three.js using their OBJLoader and OBJExporter) from an OBJ with quads. Could it be that the axis orientation that is expected by `obj2gltf` (or glTF itself) is different from what Three.js uses?

Would you have any other idea of why only some faces get flipped?