Small optimizations
Small optimizations

Avoids detecting if token is stale if auto-detection is turned off.
It now asynchronously reads the content of the response when detecting
if token is stale.

aarondcoleman/Fitbit.NET,AlexGhiondea/Fitbit.NET,WestDiscGolf/Fitbit.NET,amammay/Fitbit.NET,AlexGhiondea/Fitbit.NET,amammay/Fitbit.NET,AlexGhiondea/Fitbit.NET,WestDiscGolf/Fitbit.NET,aarondcoleman/Fitbit.NET,aarondcoleman/Fitbit.NET,WestDiscGolf/Fitbit.NET,amammay/Fitbit.NET