trailing-slash for local URL

WP common configuration adds a trailing-slash to URL.
`trailingslashit` is in charge for this.

When redirecting to a local URL, omitting the trailing-slash incur another redirect for the purpose of adding the slash.
The current code always drop that slash implying **two** redirections.

Start by locating the redirect handling for local URLs and inspect how trailingslashit is used or bypassed. Reproduce a local URL without a trailing slash, then confirm the redirect preserves the slash so WordPress does not issue a second redirect.

Written by the indexing model from the issue text.