Based off this question, you could try redefining process() in ActionController::TestCase from https://github.com/rails/rails/blob/32395899d7c97f69b508b7d7f9b7711f28586679/actionpack/lib/action_controller/test_case.rb.
Here is my workaround though.
describe FooController do let(:defaults) { {format: :json} } context 'GET index' do let(:params) { defaults } before :each do get :index, params end # ... end context 'POST create' do let(:params) { defaults.merge({ name: 'bar' }) } before :each do post :create, params end # ... end end