Test failures indicate that this operation doesn't always take effect
immediately:
https://travis-ci.org/hashicorp/terraform/builds/103764466
Add a simple poll to retry a few times until it does.
```
--- PASS: TestAccMailgunDomain_Basic (1.51s)
```
Verified that this does the trick by looping the test and watching the
logs for the retry behavior to kick in.
The Mailgun provider was relying on an old behavior of
`ResourceData.Set` that would allow nested access to
maps. We now just build up our own maps like sane people.
Before all providers were using the helper.Schema approach the helper
function had these names. Now they all use names consistent with the Go
naming conventions except for these last few…
Paul Hinze