Interfaces for Client and AsyncClient

[alexanderkolbyoshkin]

How about extracting interfaces from the Client and AsyncClient classes? This would allow to mock them in unit tests without additional wrappers.

[bzaar]

The recommended way of mocking HttpClient is... not to mock it. :) Instead, pass a mock implementation of HttpMessageHandler to HttpClient constructor. See https://stackoverflow.com/a/36427274/1291717