Making `bbox` compatible with `sf::st_bbox`

[Rafnuss]

Would be nice to accept a bbox computed from sf
Example:

get_stadiamap(st_bbox(sf_object), zoom = 13, maptype = "alidade_smooth") |> ggmap()

Currently we have:

> bbox = c(xmin = 39.758428, ymin = -2.982270, xmax = 39.845852, ymax = -2.855576 )
> get_stadiamap(bbox, zoom = 13, maptype = "alidade_smooth") |> ggmap()
ℹ © Stadia Maps © Stamen Design © OpenMapTiles © OpenStreetMap contributors.
Error in get_stamen_url(maptype = maptype, zoom = zoom, x = row["x"],  : 
  is.wholenumber(x) || !(0 <= x && x < 2^zoom) is not TRUE

I think we could simply remove the if statement:

ggmap/R/get_stadiamap.R

Line 185 in 8b12beb

if(is.null(names(bbox))) names(bbox) <- c("left","bottom","right","top")

names(bbox) <- c("left","bottom","right","top")