[FIX] web_advanced_search: `undefined` in x2m fields

Before this patch, when searching with the "equals to" operator in any
x2many field, the searched parameter was always `undefined`.

The problem was that the underlying field manager implementation was
treating those fields as x2many, while the widget used was the `one2many`
one.

This patch simply mocks the underlying fake record to make think that
any relational field is always a `one2many`. This sets all pieces in
place and makes the field manager work as expected, and thus you can
search as expected too.

web_advanced_search/static/src/js/web_advanced_search.js

@@ -178,6 +178,10 @@ odoo.define("web_advanced_search", function (require) {
             };
             // See https://stackoverflow.com/a/11508530/1468388
             params.fields[this.field.name] = _.omit(this.field, "onChange");
+            if (this.field.type.endsWith("2many")) {
+                // X2many fields behave like m2o in the search context
+                params.fields[this.field.name].type = "many2one";
+            }
             params.fieldsInfo.default[this.field.name] = {};
             // Emulate `model.load()`, without RPC-calling `default_get()`
             this.datapoint_id = this.model._makeDataPoint(params).id;