[FIX] base: correctly keep report values

When rendering a report (`render_qweb_html` for example), one can pass `data`
(a dict of values, used for the template rendering).

This dict was overriden in `_get_rendering_context` if the method
`_get_report_values` on the report model was not correctly written.

This was causing an issue when rendering a report in Studio because the `studio`
key was overriden.

Now, the `data` dict is updated (instead of replaced) with the rendering context.

odoo/addons/base/models/ir_actions_report.py

@@ -705,12 +705,12 @@ class IrActionsReport(models.Model):
         report_model_name = 'report.%s' % self.report_name
         report_model = self.env.get(report_model_name)
 
+        data = data and dict(data) or {}
+
         if report_model is not None:
-            data = report_model._get_report_values(docids, data=data)
+            data.update(report_model._get_report_values(docids, data=data))
         else:
             docs = self.env[self.model].browse(docids)
-            if not data:
-                data = {}
             data.update({
                 'doc_ids': docids,
                 'doc_model': self.model,