filter duplicate header #80

Since you have a lot of possibilities when gluing together methods you can easily mess up the requests.
I did something like:

wretch(url).headers( 'content-type': 'application/json' ).post(...).json()
// one of these methods added another application/json header

and saw that in my requests I had duplicate header: Content-Type: application/json, application/json. Causing my server to reject the request.
This shouldn't be possible. Maybe worth filtering duplicate header?