swift: fix missing hash from object returned from upload

Before this fix we were reading the hash from the upload using the
string "ETag", however the go runtime normalises the tag into "Etag"
so we were in fact always reading an empty string.

This bug was introduced in

aeea4430d5 swift: efficiency: slim Object and reduce requests on upload

It was spotted by the integration tests.

The fix was just to use the canonical form "Etag" instead of "ETag".

backend/swift/swift.go

@@ -1335,7 +1335,7 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 		// object has been safely uploaded
 		o.lastModified = modTime
 		o.size = size
-		o.md5 = rxHeaders["ETag"]
+		o.md5 = rxHeaders["Etag"]
 		o.contentType = contentType
 		o.headers = headers
 		if inCount != nil {