Headers class should accept 'IHeader' instances in Add function

[HoPe0211]

Hi Team,

I created a patch file with a possible improvement for replacing the Header class to IHeader interface type in Headers.Add function.
This simple change allows the client to use any kind of implementation from IHeader interface as a header item.

Please consider my patch file:

0001-Refactor-Header-classes-and-improve-async-practices.patch