XHTTP: only force trailing path slash when session/seq use path placement

[dudkin-2005]

GetNormalizedPath always appends a trailing slash to the configured path.
That slash is only meaningful when sessionID and/or seq are carried in the
path: it separates the configured base path from the appended segments and
lets the server slice them back off via req.URL.Path[len(path):].

When both sessionID and seq are placed in query/cookie/header, the path
carries no appended segments, so the forced trailing slash is unnecessary.
It also makes URLs look unnatural — a directory-style trailing slash right
before the query string, or a file path that gets a slash appended after its
extension.

This change makes the trailing slash conditional on path placement being
used for sessionID or seq. The default placement is path, so any config
that does not move both fields off the path keeps the exact previous
behaviour; only configs that move both fields off the path are affected.

Examples

Both meta fields in query

"xhttpSettings": {
  "mode": "packet-up",
  "path": "/stream",
  "sessionIDPlacement": "query",
  "seqPlacement": "query"
}

Before:

GET  /stream/?x_session=<id>
POST /stream/?x_seq=0&x_session=<id>

After:

GET  /stream?x_session=<id>
POST /stream?x_seq=0&x_session=<id>

File-like path — "path": "/stream/filename.extension", both in query:
Before → /stream/filename.extension/?x_session=<id>
After → /stream/filename.extension?x_session=<id>

Unchanged: default (path) placement — "path": "/abc":

GET  /abc/<sessionID>
POST /abc/<sessionID>/<seq>

sessionID and seq are in the path, so the trailing slash is still added — identical to the current behaviour.

Unchanged: root path — "path": "/" stays /?... in all cases.

[artur002]

By the way, you can check the patch and include this code snippet in the branch. You do realize this is an inherent shortcoming, right? Xray has always been highly flexible when it comes to traffic configuration. This looks more like an oversight.