tree fix

[ljw20180420]

• Suppress [error opening dir] by remove quote around .
  • When tree_opts is empty, tree treats "$tree_opts" as directory and fails to open it. Remove the quote fix this.
• truncate the tail / of path before passed to tree -H-H
  • When use the root path / in the example on_uri_match '/(.*)' serve_dir_or_file_from "assets", tree -H / will generate href of the form //file. //file is resolved as hostname file instead of a path. I fix it by truncate the tail /. Non-root path like /path is not affected because tree -H /path adds / at the tail of /path automatically.