use default description if reason phrase is blank

[kiftio]

What are you trying to accomplish?

I noticed that the reasonPhrase was empty for a 502 during testing. This adds a default errorDescription for that case.

Also tries to make the tests a little easier to read by adding a custom assertion class, chaining assertions fluently, and extracting a function to trigger the http error callback.

Before you deploy

• [x] I have added tests to support my implementation
• [x] I have read and agree with the contributing documentation
• [x] I have read and agree with the code of conduct documentation
• [ ] I have updated the README (if applicable).