Make FS return a redirect for directories without trailing slash

[moqmar]

Fixes #792

[erikdubbelboer]

Thanks. Can you add a test case please?

[moqmar]

I've added a test case and actually found an issue due to that :D

I'm not completely sure though if there are other functions making use of fs though that way (overriding the request URI before letting it serve a file or directory) - I haven't found any yet at least.

[kimkit]

@moqmar is it better to use origin path for redirect ?

https://github.com/valyala/fasthttp/blob/master/fs.go#L738

ctx.RedirectBytes(append(ctx.Path(), '/'), StatusFound)