Fix: Encrypted refresh tokens containing '+' were being incorrectly encoded

[Mohamad-Kredly]

Fixed an issue where an encrypted refresh token containing the '+' symbol would be sent as a space instead, failing the refresh.

Context:

I was using the SpotifySDK to auth based off of the example project and I was having problems where sometimes the token fails to refresh when using the AuthorizationCodeFlowBackend's refresh. Refreshing with the SessionManager didn't have any issues so I looked at the backend and I saw that the token sent from SpotifyAPI was being decoded as a space instead of the +.

The backend used is the one in the SpotifySDK demo projects:
https://github.com/spotify/ios-sdk/blob/master/DemoProjects/spotify_token_swap.rb

I'm unsure if this occurs on other servers and if this is instead a fault from the backend I'm using 😅

[Peter-Schorn]

Hi Mohamad,

Thanks for bringing this issue to my attention. You are, indeed, correct that Dictionary.formURLEncoded() is not properly percent-encoding the + character. After further investigation, I determined that there may be other characters that may not be encoded correctly. I have made my own changes to the Dictionary.formURLEncoded() method and will release a new version shortly.

[Peter-Schorn]

Just pushed version 4.0.1, which should fix this issue by properly percent-encoding the + character, among other characters. Please confirm it is working for you and I will close this PR.

[Mohamad-Kredly]

Just got around to testing your fix and it works, thanks!