How to set a header?

Using the client middleware, how do I set a header. I'm talking about when you do a request in this way:

```
return {
types: [LOGOUT, LOGOUT_SUCCESS, LOGOUT_FAIL],
promise: (client) => client.get('/logout')
};
```

Thanks.

Start by locating the client middleware and the implementation behind client.get('/logout') to determine how request options are passed. Document the supported way to set a header and verify the example against the middleware's existing request behavior.

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