Density option not working

I come here from the [PDF2Pic](https://github.com/yakovmeister/pdf2image) library, where the `density` option is broken. For some reason, it does not work when using graphicsmagick with PNGs.

After some testing, I believe that bug is inherited from this library.

Link to original issue: https://github.com/yakovmeister/pdf2image/issues/162