credentials not implemented for fetch in some environments such as cfw

When trying to use redaxios on CloudFlare Workers it will throw the following error:
```
 The 'credentials' field on 'RequestInitializerDict' is not implemented.
```
As the default is same-origin for credentials this could probably be worked around by not passing a credentials property when `withCredentials` is set to false:
https://developer.mozilla.org/en-US/docs/Web/API/Request/credentials
