axis labels with pixel units get shifted in export

A minimal working example is below:

```Matlab
close all;
xlabel('x','units','pixels')
ylabel('y','units','pixels')
export_fig('test.png')
```

The result is as follows:


The x and y labels are shifted, when they should remain in the middle of their respective axes.

I am using MATLAB 2015b for Mac and the most recent version of `export_fig` (as of March 30, 2016).