Possibly related to #135

Describe the bug
When deleting/overwriting a global option in one instance of useFetch, the global options are then permanently modified across all instances.

Following the example in the README:

 // let's say for this request, you don't want the `Accept` header at all const { loading, error, data: todos } = useFetch(globalOptions => { delete globalOptions.headers.Accept return { data: [], ...globalOptions } }, []) // onMount 

This line:
delete globalOptions.headers.Accept

Will delete the Accept header for all useFetch instances across an app.

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To Reproduce
Steps to reproduce the behavior:

1. Go to Login page
2. Click on "Login" button
3. Open the console - the Todo useFetch options do not have the interceptors.request property that is defined in the global options passed to the Provider.

Expected behavior
I would expect that modifying the global options in an instance of useFetch would create a local copy and would not affect the global options in any other instances.