Harre/net48 support and sample

[hartmark]

This library targets .net standard 2.0 so it can be used in a .net framework 4.8 project.

However if doing so you need to add your own middleware to get the headers properly set.

I have added a sample test-project that uses .net 4.8 and is wrapping CorrelationIdMiddleware as a HttpMessageHandler so we can utilize the functionality of this library.