check for trailing slash in /foo and /foo/bar

This is a simple way of doing what you want:

when HTTP_REQUEST {
    if {not ([HTTP::path] ends_with "/")}{
        HTTP::respond 301 noserver Location "${protocol}:://[HTTP::host][HTTP::path]"
    }
}

One thing to mention though your original iRule used HTTP::uri, that will contain query parameters if they exist which would make your redirect look like this, http://some.domain.com/originalUri?parameter1=xyz/.

If you want to account for query parameters you could do this:

when HTTP_REQUEST {
    if {not ([HTTP::path] ends_with "/")}{
        if {[HTTP::query] ne ""}{
            set myuri "[HTTP::path]/?[HTTP::query]"
        }
        else {
            set myuri "[HTTP::path]/"
        }
        HTTP::respond 301 noserver Location "${protocol}:://[HTTP::host]$myuri"
    }
}