get_map with source osm still uses get_googlemaps

## Reproducible Example

The following code uses the OSM source, but in the back it still uses Google Maps to generate the bounding box. I think it is cleaner to just throw an error that this isn't supported.

``` r
ggmap::get_map(location = c(lon=5, lat=3), source='osm')
#> Source : https://maps.googleapis.com/maps/api/staticmap?#center=3,5&zoom=10&size=640x640&scale=2&maptype=terrain&key=xxx
#Error in aperm.default(map, c(2, 1, 3)) :
# invalid first argument, must be an array
#In addition: Warning message:
#In get_googlemap(center = location, zoom = zoom, filename = filename) :
# HTTP 400 Bad Request
```