Fix wrong XML format for supportedlock prop

[Vigilans]

As per specification from http://www.webdav.org/specs/rfc4918.html#PROPERTY_supportedlock :

PROPERTY_supportedlock should have following format:

<D:supportedlock> 
  <D:lockentry> 
    <D:lockscope><D:exclusive/></D:lockscope> 
    <D:locktype><D:write/></D:locktype> 
  </D:lockentry> 
  <D:lockentry> 
    <D:lockscope><D:shared/></D:lockscope> 
    <D:locktype><D:write/></D:locktype> 
  </D:lockentry> 
</D:supportedlock> 

While that returned by webdav-handler-rs is:

<D:supportedlock> 
  <D:lockentry> 
    <D:lockscope><D:exclusive/><D:write/></D:lockscope> 
  </D:lockentry> 
  <D:lockentry> 
    <D:lockscope><D:shared/><D:write/></D:lockscope> 
  </D:lockentry> 
</D:supportedlock> 

This made webdav libraries like https://github.com/lookfirst/sardine and https://github.com/thegrizzlylabs/sardine-android report error on parsing XML returned by webdav-handler-rs (thus android apps using this library will fail to use webdav server of webdav-handler-rs). This PR fixes the format by specfication.

This PR is also opened in miquels/webdav-handler-rs#24. I also opened several other PR there (miquels/webdav-handler-rs#11 and miquels/webdav-handler-rs#25), but are related to filesystem quota feature that is mainly relevant with webdav-server-rs binary.