Fix access_denied issue

[jblafage]

Hello,
I had access_denied error when I wanted to authenticate my application to our Ping Federate instance.
After some modifications on your code, it works fine.

I propose you the following modifications:

• Change the way to use HttpClient object: as it is a disposable object, it's recommended to instantiate it only when you need to perform a call. So, I have defined methods for GET and POST calls on handler side instead of having a global httpclient at middleware level
• I have also changed the UserName value when you retrieve user object, in order to map sub property when preferred_username property is not defined (our instance is configured to get only sub property)
• I have changed the way to get assembly version, because I had an issue to clean assembly each time I compile after running the tool to generate NuGet package