access-token header is blank when token isn't changed yet.

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* **Version**: 1.2.1, using redux-auth-token

the token header (defaults to "access-token") is blank if the token doesn't change. Not blank if the token does change.

With the configuration `change_headers_on_each_request = true` old libraries like redux-auth-token assume that a token will come in with every request. Instead it is only set after the old token expired. There are a bunch of requests for which the token is blank, and the library just expects the client to use the lat token it provided.

This is definitely something that can be worked around (though had to rip open the library) but would be really ideal if we could just not return blank headers. Just return the same token if it isn't changed, and return a new one if appropriate.

Sidenote: Shouldn't every request get a new token anyways?

Start by tracing the access-token header generation around the change_headers_on_each_request configuration and reproduce the behavior with redux-auth-token. Done means unchanged tokens no longer produce blank headers, while a new token is returned when appropriate; verify the request and response behavior with the project's existing tests.

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