RUBY-1082 Account for when write concern is nil in GridFS write stream

Author: estolfo

lib/mongo/grid/stream/write.rb

@@ -157,7 +157,8 @@ def files_collection
           end
 
           def with_write_concern(collection)
-            if collection.write_concern.options == write_concern.options
+            if write_concern.nil? || collection.write_concern.nil? ||
+                collection.write_concern.options == write_concern.options
               collection
             else
               collection.client.with(write: write_concern.options)[collection.name]

spec/mongo/grid/stream/write_spec.rb

@@ -66,6 +66,17 @@
       end
     end
 
+    context 'when the fs does not have a write concern' do
+
+      let(:fs) do
+        authorized_client.with(write: nil).database.fs
+      end
+
+      it 'uses the write concern default at the operation level' do
+        expect(stream.write(file).closed?).to eq(false)
+      end
+    end
+
     context 'when provided options' do
 
       context 'when provided a write option' do