Inconsistent default central meridian of projection in modern mode

I have found that the central meridian is not set consistently when creating plots in modern mode:

Using an example grid file, e.g.
[gridfile.zip](https://github.com/GenericMappingTools/GMT.jl/files/7534977/gridfile.zip)

```
using GMT

gmtbegin("test_grdimage.png")
coast(
region = :g,
projection = :Robinson,
frame = :auto,
land = :dimgray,
shore = :thinner,
)
grdimage!(
"dir.2020.1.GeeK2007.6m.nc",
cmap = "cyclic",
nan_alpha = true,
)
gmtend(show = true)
```
produces:


It is fixed by `projection = (; name = :Robinson, center = 180)` for example. So probably `grdimage` is setting a different default value. Low-priority issue since there is an easy workaround. Only affects modern mode.