Colorbar ignores the GMTcpt input if non keywords as cmap=cpt

If a ``current_cmap`` exists, then that's was is picked instead of ``geoid_cpt`` in
```
colorbar!(geoid_cpt, pos=(anchor=:TR, inside=true, ...
```
Current solution is to be explicit as in
```
colorbar!(cmap=geoid_cpt, pos=(anchor=:TR, inside=true, ...
```
but this is not fully satisfying.