When helping a member in the discord channel #help-with-circuitpython, I had made the mistake of thinking that this library didn't automatically deal with turning a dict into a urlencoded string. I had them convert it by hand and try and it worked.

As it turns out - it does, but also sends the header Content-Type: application/x-www-form-urlencoded, which some systems (Spotify as one) can't handle getting the header twice.

I would like to open a PR to check the header for an existing Content-Type before adding it's own (both for urlencoded and json). It would implement a method to check for an existing value since we would also want to check for casing mis-matches.