give a little more guidance in setting base URL

[jeanetteclark]

Loving the package, thank you for your hard work!

Based on my own experience setting up my credentials, and a flurry of related issues (#173, #179, #177, #184, #193), I think that it might be a nice improvement if assert_base_url does a little more checking to help guide users to the correct URL structure.

an assert_that the URL does not start with https:// statement would be the most helpful, and perhaps mention of no trailing slash in the existing statement for endsWith may also be useful.

I can make a quick PR if this is of interest

[juliasilge]

That would be super @jeanetteclark! 🙌 A PR to improve assert_base_url() would be most welcome.

[juliasilge]

Closed in #258