storage/curl: request the "resourcetype" property to fix update

Without requesting the property, "good" WebDAV servers would not send
it, and so MPD could never recognize a directory, failing the database
update.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

src/storage/plugins/CurlStorage.cxx

@@ -277,6 +277,7 @@ public:
 		request.SetOption(CURLOPT_POSTFIELDS,
 				  "<?xml version=\"1.0\"?>\n"
 				  "<a:propfind xmlns:a=\"DAV:\">"
+				  "<a:prop><a:resourcetype/></a:prop>"
 				  "<a:prop><a:getcontenttype/></a:prop>"
 				  "<a:prop><a:getcontentlength/></a:prop>"
 				  "</a:propfind>");