[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.