[flutter_appauth_platform_interface] Fixes error on casting the tokenAdditionParameters property from calling token method

[svendroid]

Fixes #86

[MaikuB]

I've changed the title to add more details. Thanks a lot for this. I guess you're the first one to have actually used this method for since no one else had picked up on this ^^;

[svendroid]

Thanks for the quick replies. I'm not really using the map, I stumbled about it because I logged all properties during development, but maybe someone else will in the future. ;)

[MaikuB]

Funny enough, I had just spoken to someone about how it's great to see pull requests get submitted so it's great timing ;)