Fix firefox tests

[jasonpaulos]

• Switched to an Ubuntu base image for the docker test runner since installing Firefox on Debian proved to be more difficult.
• Firefox tests were failing because of the issue I pointed out here: Each executed script has a different global scope mozilla/geckodriver#1798. I implemented a workaround to this by using the helper functions makeUint8Array and makeEmptyObject and by fixing how we check for arrays in the SDK.
• I also found that the code for POST and DELETE requests throw a Response object instead of an Error if an error occurs, so I fixed this to improve the error message that gets displayed.

[EvanJRichard]

Nice, I like this. I think there was some reason for throwing e.response rather than e, but I can't find that reason documented anywhere. So: LGTM!