fix(grid-store): add missing callback

Author: Jessica Lord

lib/gridfs/grid_store.js

@@ -452,7 +452,9 @@ var writeFile = function(self, file, callback) {
               self.currentChunk = chunk;
 
               if (offset >= stats.size) {
-                fs.close(file);
+                fs.close(file, function(err) {
+                  if (err) return callback(err);
+                });
                 self.close(function(err) {
                   if (err) return callback(err, self);
                   return callback(null, self);