RefreshToken should only try to parse the token in a succesful response

[mxa0079]

[DefaultTokenManager.RefreshToken()](https://github.com/aarondcoleman/Fitbit.NET/blob/async-portable/Fitbit.Portable/OAuth2/DefaultTokenManager.cs#L11) should only try to parse during a successful reply.

We are currently parsing in all cases, which is in exceptions when parsing.