Also use content checksum for ETag calculation

Timestamp is not enough, as it might be the same
for two consequent PUTs.

lib/remote_storage/swift.rb

@@ -156,7 +156,7 @@ module RemoteStorage
 
       if update_metadata_object(user, directory, key, metadata)
         if metadata_changed?(existing_metadata, metadata)
-          update_dir_objects(user, directory, timestamp)
+          update_dir_objects(user, directory, timestamp, checksum_for(data))
         end
 
         server.headers["ETag"] = %Q("#{res.headers[:etag]}")
@@ -166,6 +166,10 @@ module RemoteStorage
       end
     end
 
+    def checksum_for(data)
+      Digest::MD5.hexdigest(data)
+    end
+
     def delete_data(user, directory, key)
       url = url_for_key(user, directory, key)
 
@@ -316,9 +320,9 @@ module RemoteStorage
       true
     end
 
-    def update_dir_objects(user, directory, timestamp)
+    def update_dir_objects(user, directory, timestamp, checksum)
       parent_directories_for(directory).each do |dir|
-        etag = etag_for(dir, timestamp)
+        etag = etag_for(dir, timestamp, checksum)
 
         key = "rs:m:#{user}:#{dir}/"
         metadata = {e: etag, m: timestamp}