Consider not mutating provided opts object

[celayale]

We are facing an issue when migrating from node-fetch to @zeit/fetch due to the fact that some of the properties in provided opts object are mutated before calling the decorated fetch function.

We are having issues mainly with headers, but I see the body is overwritten too.

Maybe it would be a good idea to copy provided opts and mutate the copy before passing to the wrapped fetch instead? It would remove these unexpected side effects.

I think I could try to provide the fix if you are ok with it.

[celayale]

The implementation would be quite simple. Let me know if you want me to create the PR.

https://github.com/zeit/fetch/compare/master...celayale:feature/avoid-mutating-opts?expand=1

[cgimeno]

+1

[Ethan-Arrowood]

Thank you for the report. We are archiving this project; you can read more details here: #83