Custom dav properties with namespaces are rendered incorrectly

## Describe the bug

Custom dav properties in dav response are rendered incorrectly

## Steps to reproduce

Steps to reproduce the behavior:

https://github.com/owncloud/core/blob/076ddc7dc430e1e80d23b374d13c73f199cb398a/tests/acceptance/features/apiWebdavProperties1/setFileProperties.feature#L24

## Expected behavior
custom dav properties with namespace rendered as in oc10
```

/core/remote.php/dav/files/Alice/testcustomprop.txt






HTTP/1.1 200 OK

```
## Actual behavior
dav properties are rendered incorrectly in the xml
```

/remote.php/dav/files/Alice/testcustomprop.txt


<foo xmlns='http://bar'/>

HTTP/1.1 200 OK

```

## Setup

Please describe how you started the server and provide a list of relevant environment variables.

```console
OCIS_VERSION=git
BRANCH=master
STORAGE_FRONTEND_UPLOAD_DISABLE_TUS=false
```

## Additional context
Add any other context about the problem here.