[FIX] mrp: create mrp.document through mrp.eco

How to reproduce:
  - Open PLM app, open an Engineering Change Order;
  - Click on "Update Documents";
  - Try to upload a document -> Traceback.

As we did not cast res_id as integer, the check method of ir.attachment
crashes when accessing company_id on a malformed recordset of ECO
because the domain generated by _filter_access_rules_python is:

  `['|', ('company_id', '=', False), ('company_id', 'in', [2, 1])]`

This route is also used when adding attachment to a `product.product`.
In this case the check didn't work either but somehow passed because no
domain is generated by `_filter_access_rules_python`.
The following snippet shows that if there was one, it would also crash:

```
In [16]: records
Out[16]: product.product('25',)

In [17]: records.stock_move_ids
Out[17]: stock.move()

In [18]: self.env['product.product'].browse(25)
Out[18]: product.product(25,)

In [19]: self.env['product.product'].browse(25).stock_move_ids
Out[19]: stock.move(41, 13)
```

task-2223153

closes odoo/odoo#48515

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>

addons/mrp/controller/main.py

@@ -25,7 +25,7 @@ class MrpDocumentRoute(http.Controller):
                 request.env['mrp.document'].create({
                     'name': ufile.filename,
                     'res_model': kwargs.get('res_model'),
-                    'res_id': kwargs.get('res_id'),
+                    'res_id': int(kwargs.get('res_id')),
                     'mimetype': mimetype,
                     'datas': base64.encodebytes(ufile.read()),
                 })