Fix deprecated Faraday basic_auth

[estromlund]

Hi there 👋

We're using your library in our stack and have noticed that there are now warnings being thrown for usage of deprecated Faraday methods.

Specifically:

WARNING: `Faraday::Connection#basic_auth` is deprecated; it will be removed in version 2.0.
While initializing your connection, use `#request(:basic_auth, ...)` instead.
See https://lostisland.github.io/faraday/middleware/authentication for more usage info.

This PR updates the usage of basic_auth to be in line with the suggestion at that link.

I searched for other usages of basic_auth in here but this seems to be the only instance.

The specs ended up getting a little funky since this new approach adds the auth at the middleware level and not on the connection itself. This means that the Authorization headers don't exist on the connection, but are added once the request is in flight. I modified the specs to use an approach used by Faraday itself.

Please let me know if you have any questions and thanks for the consideration!

[ajrice6713]

Hey @estromlund!

Thanks for taking the time to contribute to the project! We will take a look and run some tests to ensure everything looks good before merging in.

@ckoegel for visibility

[ckoegel]

Thanks for also including a test! @estromlund
This also revealed an issue with our activesupport dependency which will be fixed alongside this change. Everything looks great to me.