mogrify resolution of png image: error when using PixelsPerInch unit

Hello all,

When I use mogrify to set the resolution in PixelsPerInch of an png image,
the numbers are set but the unit is PixelsPerCentimeter.

$ mogrify -density 200x200 -units PixelsPerInch image.png
$ identify -verbose image.png | grep -A 1 Resolution
  Resolution: 199.99x199.99
  Units: PixelsPerCentimeter

$ mogrify -density 75x75 -units PixelsPerCentimeter image.png
$ identify -verbose image.png | grep -A 1 Resolution
  Resolution: 75x75
  Units: PixelsPerCentimeter

$ mogrify -density 150x150 -units PixelsPerInch image.png
$ identify -verbose image.png | grep -A 1 Resolution
  Resolution: 149.99x149.99
  Units: PixelsPerCentimeter

Without specifying unit, default unit seems to be PixelsPerCentimeter, not
PixelsPerInch as in the documentation

$ mogrify -density 200x200 image.png
$ identify -verbose image.png | grep -A 1 Resolution
  Resolution: 200x200
  Units: PixelsPerCentimeter

It is not identify that is wrong, Gimp always confirms it's values.