Implement Remote::FS#sftp

spec/producer/core/remote/fs_spec.rb

@@ -10,5 +10,25 @@ module Producer::Core
         expect(fs.instance_eval { @remote }).to be remote
       end
     end
+
+    describe '#sftp', :ssh do
+      before { sftp_story }
+
+      it 'builds a new SFTP session' do
+        expect(remote.session.sftp).to receive(:connect)
+        fs.sftp
+      end
+
+      it 'returns the new SFTP session' do
+        session = double('session')
+        allow(remote.session.sftp).to receive(:connect) { session }
+        expect(fs.sftp).to be session
+      end
+
+      it 'memoizes the FS' do
+        allow(remote.session.sftp).to receive(:connect) { Object.new }
+        expect(fs.sftp).to be fs.sftp
+      end
+    end
   end
 end