Make normalize_path configurable to allow for trailing slashes

[alper]

I think paths should be normalised to end with a slash.

See how (and why) Django does this:

• https://stackoverflow.com/a/42213107/102315
• https://docs.djangoproject.com/en/dev/ref/settings/#std-setting-APPEND_SLASH

Since tower-http/src/normalize_path.rs has already gone in the other direction, I guess the best way forward would be to make it configurable so that those who prefer trailing slashes can get them?

I'll happily make the change.