webdav: fix tests after go1.20 upgrade

Before this change we were sending webdav requests to the go http
FileServer. In go1.20 these (rightly) started returning errors which
caused the tests to fail.

The test has been changed to properly mock up an About query and
response so an end to end test of adding headers is possible.

backend/webdav/webdav_internal_test.go

@@ -24,15 +24,23 @@ var (
 // prepareServer the test server and return a function to tidy it up afterwards
 // with each request the headers option tests are executed
 func prepareServer(t *testing.T) (configmap.Simple, func()) {
-	// file server
-	fileServer := http.FileServer(http.Dir(""))
-
-	// test the headers are there then pass on to fileServer
+	// test the headers are there send send a dummy response to About
 	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		what := fmt.Sprintf("%s %s: Header ", r.Method, r.URL.Path)
 		assert.Equal(t, headers[1], r.Header.Get(headers[0]), what+headers[0])
 		assert.Equal(t, headers[3], r.Header.Get(headers[2]), what+headers[2])
-		fileServer.ServeHTTP(w, r)
+		fmt.Fprintf(w, `<d:multistatus xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:oc="http://owncloud.org/ns" xmlns:nc="http://nextcloud.org/ns">
+<d:response>
+ <d:href>/remote.php/webdav/</d:href>
+ <d:propstat>
+  <d:prop>
+   <d:quota-available-bytes>-3</d:quota-available-bytes>
+   <d:quota-used-bytes>376461895</d:quota-used-bytes>
+  </d:prop>
+  <d:status>HTTP/1.1 200 OK</d:status>
+ </d:propstat>
+</d:response>
+</d:multistatus>`)
 	})
 
 	// Make the test server
@@ -68,7 +76,7 @@ func TestHeaders(t *testing.T) {
 	f, tidy := prepare(t)
 	defer tidy()
 
-	// any request will do
+	// send an About response since that is all the dummy server can return
 	_, err := f.Features().About(context.Background())
 	require.NoError(t, err)
 }