Change *http.Transport to http.RoundTripper

[radeksimko]

https://golang.org/pkg/net/http/#RoundTripper

This will allow anyone to pass through custom transport implementations, such as logging transport we use over in Terraform:

https://github.com/hashicorp/terraform/blob/master/helper/logging/transport.go

*http.Transport implements this interface so this should not break anything.

I tried running tests via make test but they mostly fail (on master) for me. It may be just different environment, I don't know - I hope my changes didn't break anything. 🤞

[michaelklishin]

@radeksimko there are notes on how to set up a RabbitMQ node for running this client's tests. I'm happy to run the suite on your behalf.

I suspect we need to bump major version to go with this change?

[michaelklishin]

The test suite is happy:

=== RUN   TestRabbitHole
Running Suite: Rabbithole Suite
===============================
Random Seed: 1551039226
Will run 66 of 66 specs

••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
Ran 66 of 66 Specs in 67.267 seconds
SUCCESS! -- 66 Passed | 0 Failed | 0 Pending | 0 Skipped

[michaelklishin]

Thank you!