[FIX] core: converse existing context in JSON-RPC

Define the following controller:

    @route('/get-context', auth='public', type='json', website=True):
        return request.env.context

Access that route via JSON-RPC providing a context:

    this._rpc({
        route: '/get-context',
        params: {
            context: {"a": 1}
        }
    })

Since httpocalypse, it replaces the context entirely instead of updating
it. i.e. before you would get along those lines:

    {"a": 1, "lang": "en_US", "tz": "Europe/Brussels", "uid": 2}

but since you get instead (and that's wrong):

    {"a": 1}

Note that after this commit, this merging-context behavior will be the
same whether or not website=True is defined (while it was only there for
website=True before).

Discovered via opw-2885948

X-original-commit: c72768ff31efad71caab0763b3e8d141c9460a55
Part-of: odoo/odoo#107127