Update Documentation for testing

[Obiwancodi]

The flutter testing cookbook recommends to use http.client rather than http static methods such as post or get for testing.
https://flutter.dev/docs/cookbook/testing/unit/mocking
Can you update the documentation to mention this? Thanks

[natebosch]

Is this in the README? I think we could take a PR for this.

[arowolojoshuao]

I have raised PR to update the documentation for this issue