webdav: permit redirects on PROPFIND for metadata

The WebDAV implementation already permits redirects on PROPFIND for
listing paths in the `listAll` method but does not permit this for
metadata in `readMetaDataForPath`.  This results in a strange experience
for endpoints that heavily use redirects -

```
rclone lsl endpoint:
```

functions and lists `hello_world.txt` in its output but

```
rclone lsl endpoint:hello_world.txt
```

Fails with a HTTP 307.

The git history for this setting indicates this was done to avoid
an issue where redirects cause a verb change to GET in the Go HTTP
client; it does not appear to be problematic with HTTP 307.

To fix, a new `CheckRedirect` function is added in the `rest` library
to force the client to use the same verb across redirects, forcing this
for the PROPFIND case.

lib/rest/rest.go

@@ -215,6 +215,22 @@ func ClientWithNoRedirects(c *http.Client) *http.Client {
 	return &clientCopy
 }
 
+// PreserveMethodRedirectFn is a CheckRedirect function that
+// preserves the original HTTP method on redirects.
+//
+// By default Go's http.Client changes the method to GET on 301, 302,
+// and 303 redirects. This function overrides that behaviour so the
+// original method (e.g. PROPFIND being preserved across a 307) is kept.
+func PreserveMethodRedirectFn(req *http.Request, via []*http.Request) error {
+	if len(via) >= 10 {
+		return errors.New("stopped after 10 redirects")
+	}
+	if len(via) > 0 {
+		req.Method = via[0].Method
+	}
+	return nil
+}
+
 // Do calls the internal http.Client.Do method
 func (api *Client) Do(req *http.Request) (*http.Response, error) {
 	return api.c.Do(req)