Update partial from session with more recent values from kwargs

If I redirect from the connect pipeline to a login page which on success redirects back to complete the pipeline, the `user` kwarg retains its previously set value of `None`, when I expect it to be replaced with the logged in user. This fix handles my use case, and it feels consistent—treating the outcome of the redirect as another stage in the pipeline, overwriting previously set kwargs—but I'm not sure if this breaks existing use cases that aren't covered by the tests.