Fix drawable:set_bgimage()

When called with the file name of an image, this function failed to turn that
file name into a cairo surface.

Fixes: https://github.com/awesomeWM/awesome/issues/954
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/drawable.lua

@@ -237,6 +237,9 @@ end
 -- as arguments. Any other arguments passed to this method will be appended.
 -- @param image A background image or a function
 function drawable:set_bgimage(image, ...)
+    if type(image) ~= "function" then
+        image = surface(image)
+    end
 
     self.background_image = image
     self.background_image_args = {...}