Fix bug on Node.js < 12

[Pimm]

Inspecting objects (payments, customers, subscriptions, etc.) would throw an error on Node.js < 12, because the inspect.custom implementation relied on both Object.entries and Object.fromEntries.

[edorivai]

Approved the change, because the change itself looks good, but did you check for other occurrences of Object.* functions that are not supported by Node <12?

[Pimm]

I just checked. The only other functions we're using are Object.assign and Object.create, so we should we OK.

We do test on Node.js 6. It just turns out there is no .toString() in the test suite. Come to think of it, I might as well add that.