psconvert fails to properly reference a Mercator map

See [forum post](https://forum.generic-mapping-tools.org/t/bug-in-psconvert-to-geotiff/867) for context.

Wrong
```
gmt psbasemap -JM-81/46/15 -B0.1 -R-81.3/-80.9/46.6/46.8 -BneSW -P > lixo.ps

psconvert -W+g lixo.ps

gdalinfo lixo.tiff
...
Corner Coordinates:
Upper Left ( -26421.455, 4090905.055) ( 81d14'14.45"W, 34d38'27.40"N)
Lower Left ( -26421.455, 4066620.536) ( 81d14'14.45"W, 34d27'37.61"N)
Upper Right ( 9180.188, 4090905.055) ( 80d55' 3.12"W, 34d38'27.40"N)
Lower Right ( 9180.188, 4066620.536) ( 80d55' 3.12"W, 34d27'37.61"N)
Center ( -8620.634, 4078762.796) ( 81d 4'38.79"W, 34d33' 2.68"N)
```

Right
```
gmt psbasemap -JM15 -B0.1 -R-81.3/-80.9/46.6/46.8 -BneSW -P > lixo.ps

psconvert -W+g lixo.ps

gdalinfo lixo.tiff
...
Corner Coordinates:
Upper Left ( -26837.295, 5878880.358) ( 81d20'27.90"W, 46d48'12.27"N)
Lower Left ( -26837.295, 5843982.022) ( 81d20'27.90"W, 46d35'15.75"N)
Upper Right ( 24324.439, 5878880.358) ( 80d52'53.36"W, 46d48'12.27"N)
Lower Right ( 24324.439, 5843982.022) ( 80d52'53.36"W, 46d35'15.75"N)
Center ( -1256.428, 5861431.190) ( 81d 6'40.63"W, 46d41'44.40"N)
```

Is is still the Mercator with a true scale lat away form equator?