Describe the bug

在当前代码中，HEAD 请求的目标只能是文件，当 HEAD 一个目录时，会返回 404
Cloudreve/pkg/webdav/webdav.go

Lines 236 to 239 in 4d7b868

| | exist, file:=fs.IsFileExist(reqPath) |
| | if!exist { |
| | returnhttp.StatusNotFound, nil |
| | } |

To Reproduce

Steps to reproduce the behavior:
HEAD http://example.com/dav/

Expected behavior

return 0, nil

Additional context

相关方法的 RFC 标准：
http://www.webdav.org/specs/rfc4918.html#rfc.section.9.4