fix: don't remove trailing slash when using path.Clean and path.Join

[xqqp]

path.Join and path.Clean remove trailing slashes. This is an issue when the upstream server expects them to be contained in the URL path.

Example:
Upstream expects: http://host/path/1/
Client does request to http://host/path/1/ but Oathkeeper modifies the path to http://host/path/1.

With this PR, trailing slashes are added after using the two functions, if a trailing slash existed before.

Related issue(s)

#1266

Checklist

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Further Comments

Tested locally.

[CLAassistant]

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