Improve delegating handlers usability (call to base inside CEs scope)

**Is your feature request related to a problem? Please describe.**
https://wizardsofsmart.wordpress.com/2014/05/13/how-to-use-base-sendasync-in-f-delegatinghandler/
The issue is that without this trick is hard to use delegating handlers in F# aspnetcore, and it's just a very common scenario to customize httpClient in aspnetcore.

**Describe the solution you'd like**

would be nice if the normal approach of calling base inside a CE would work, like inside task CE or async CE

**Describe alternatives you've considered**

I have used the approach proposed in the article