windows cache path

[nikolajiang]

in file proxy/route.go, use path.Dir(file) to get the dir of cache file;
but in windows it will just return ".",
because the path separator in path.Dir(path string) is "\" , and the path separator in the file is "/".
it may replace path.Dir(file) with filepath.ToSlash(filepath.Dir(file)).

[oiooj]

@nikolajiang Can you help send a PR for this issue?