added check access token validity before refresh

[fbarresi]

I use a RefreshTokenDelegatingHandler to gather a bearer token from a refresh token in my application.
However, I noticed in my code that reusing the handler (rather than instantiating it again) leads to error replies due to the use of an expired token.

This seems to be because the validity of the token after a refresh is never evaluated in successive usages, and the RefreshTokenDelegatingHandler attempts to use the expired token anyway.

In general, I think evaluating token expiration (when known) should be a common function of this handler. Additionally, reusing the handler instead of recreating it would minimise the number of requests to the IDP.