grdproject with -Js looses the true scale parameter when converting to PROJ4

For example the UPS proj
```
gmt grdproject -Js0/90/81.11451786859362545/1:1 -C -F ...
```
will store a wrong proj4 string
```
grdproject [INFORMATION]: Proj4 string to be converted to WKT:
+proj=stere +lat_0=90 +lon_0=0 +k=1 +x_0=0 +y_0=0 +units=m +a=6378137.000 +b=6356752.314 +ellps=WGS84 +datum=WGS84 +units=m +no_defs
```