Restore response.stream in render_to_string with ActionController::Live

Fix `render_to_string` in `ActionController::Live` to not break `response.stream`;
By wrapping `render_to_string` method to restore `response.stream` too.

Because `render_to_string` modifies response_body temporally, but
it doesn't restore `response.stream`.

This will work properly by this fix:

    class FooController < ActionController::Base
      include ActionController::Live

      def streaming
        response.stream.write "foo\n"

        # This breaks existing response.stream before
        render_to_string(text: 'zomg')

        response.stream.write "bar\n"
        response.stream.close
      end
    end