bugfix the (non)existence of a slash when staticFolder is set

[coderofsalvation]

so, as @reddec already expected the staticDir crashed when no trailing space existed in an URL.
This fix handles all possible situations:

situation	input	output
without slash	e87-ecf98-ecf998-ef98
with slash	e87-ecf98-ecf998-ef98/
with slashes	e87-ecf98-ecf998-ef98/foo/bar	foo/bar

the first two scenarios will correctly fallback to serving index.html

[reddec]

@coderofsalvation thanks for the idea, I used it as basis in d2e405c . Could you please confirm that it will solve your case?

[coderofsalvation]

yup that works!
Btw. I've replaced/force-pushed this branch with another fix: it resolves index.html in any directory now (see commit)
This seems to be the default behaviour on many static-file http servers.

[reddec]

Nice idea! Left few comments

[reddec]

It feels like Readdir here is overkill 😺

Could you please use something like this

if _, err := os.Stat("/path/to/whatever"); errors.Is(err, os.ErrNotExist) {
  // path/to/whatever does not exist
}

Also, for path concatenation it's better to use filepath.Join - it will take care of few corner cases