Order of operations: contourf, central_longitude=180; missing data at 180

I think projection should be handled after contours is called, or else there's a strip of missing data.

```
import xarray as xr
from holoviews.operation import contours
import hvplot.xarray
```

Top plot shows:
`contours(ds.hvplot.image('lon', 'lat', crs=ccrs.PlateCarree())).opts(projection=ccrs.PlateCarree(central_longitude=180)`

Bottom plot shows:
`ds.hvplot.contourf('lon', 'lat', crs=ccrs.PlateCarree(), projection=ccrs.PlateCarree(central_longitude=180))`