Filter causes duplication of query parameters

[alansemenov]

site.xml:

  <mappings>
    <mapping filter="/site/mappings/filter.js">
      <pattern>/filter</pattern>
    </mapping>
  </mappings>
</site>

/site/mappings/filter.js:

exports.filter = function (req, next) {
    log.info(JSON.stringify(req.params, null, 4));
    return next(req);
};

A part added to the default site's template:

exports.get = function (req, next) {
    log.info(JSON.stringify(req.params, null, 4));
    return {
        contentType: 'text/html',
        body: body
    };
};

This setup will output the following in the XP log for url http://localhost:8080/admin/site/preview/default/draft/features/filter?q=1:

16:18:35.280 INFO  com.enonic.app.features - (/site/mappings/filter.js) {
    "q": "1"
}
16:18:35.289 INFO  com.enonic.app.features - (/site/parts/my-part/my-part.js) {
    "q": [
        "1",
        "1"
    ]
}

After going through the filter, q parameter becomes an array with duplicated value.

[rymsha]

It appeared after #8210 (although, it is not a regression)

Interestingly enough we are not the first who face such issue jetty/jetty.project#2074

[rymsha]

prams from filter result are appended to params with the same name - that is causing bug in cases where filter uses next as a return value.

To do a fix we decided to changes the behavior:
prams with the same name in filter result replace params in the original request.