Fix upload ID when using S3 store with Digital Ocean Spaces (#296)

Digital Ocean Spaces (S3 compatible object storage) sends the upload id information in metadata as `upload-id` instead `upload_id`, so the `_getMetadata` method of `S3Store` returns an empty upload id.

Author: manelio

lib/stores/S3Store.js

@@ -233,6 +233,8 @@ class S3Store extends DataStore {
             .then((data) => {
                 this.cache[file_id] = Object.assign({}, data.Metadata, {
                     file: JSON.parse(data.Metadata.file),
+                    // Patch for Digital Ocean: if key upload_id (AWS, standard) doesn't exist in Metadata object, fallback to upload-id (DO)
+                    upload_id: data.Metadata.upload_id || data.Metadata['upload-id'],
                 });
 
                 return this.cache[file_id];