Don't perform trailing slash rewrite when target is directory

[jeroenpelgrims]

I messed something up in the last pull request. Before nginx would redirect every time there was a trailing slash detected on the request url. But this caused an infinite redirect loop on folders. This pull request fixes that.

Try going to the /about/ folder on a site using the code of the previous commit, it will be stuck in a redirect loop.

[inket]

Whoops 😅 Thanks for the fix!