RUBY-1082 Adjust logic

Author: estolfo

lib/mongo/grid/stream/write.rb

@@ -157,8 +157,8 @@ def files_collection
           end
 
           def with_write_concern(collection)
-            if write_concern.nil? || collection.write_concern.nil? ||
-                collection.write_concern.options == write_concern.options
+            if write_concern.nil? || (collection.write_concern &&
+                collection.write_concern.options == write_concern.options)
               collection
             else
               collection.client.with(write: write_concern.options)[collection.name]