Avoid using/changing http.DefaultClient #236

When the user of the library does not provide a custom cient,
http.DefaultClient is used, and a custom RoundTripper is set in order to
implement authentication / retries, e.g.

This can have unintended consequences and should be avoided in my opinion. Now,
every time I (or another package) uses the default client, it behaves
differently than expected.

I'd suggest to instead create a new client: cfg.HTTPClient = &http.Client{}.
This change would need to be done for every service.