get_map cannot load OpenStreetMap

Tried to use get_map to load openstreetmap, but it gives the error below:

`map <- get_map(location = 'uk', zoom = 8, source = "osm")`

> Map from URL : http://maps.googleapis.com/maps/api/staticmap?center=uk&zoom=8&size=640x640&scale=2&maptype=terrain&sensor=false
> Information from URL : http://maps.googleapis.com/maps/api/geocode/json?address=uk&sensor=false
> Error: map grabbing failed - see details in ?get_openstreetmap.
> In addition: Warning message:
> In download.file(url, destfile = destfile, quiet = !messaging, mode = "wb") :
> cannot open URL 'http://tile.openstreetmap.org/cgi-bin/export?bbox=-5.19103891796874,54.3650719770938,-1.67541391796874,56.3626958621291&scale=2800000&format=png': HTTP status was '400 Bad Request'

@dkahle Is there any effective solution to this problem? thanks.