Update DO CNAME type API request to prevent error 422

[nenadilic84]

For some reason, the DO API requires the '.' at the end of "data" in case of CNAME request
Example: {"type":"CNAME","name":"hello","data":"www.example.com."}
otherwise gives this error:
422 - Data needs to end with a dot (.)

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[njuettner]

Thank you for the PR 😄 !

Looks like other people have faced this issue as well, https://github.com/terraform-providers/terraform-provider-digitalocean/issues/58

Could you please sign the CLA first? I would also change the first comment to "422 - Data needs to end with a dot (.)"

I would also propose to add a test.

[njuettner]

@nenadilic84 thank you for signing the CLA, could you please add a test case?

[nenadilic84]

Sure, will do.

[njuettner]

@nenadilic84 did you had time to look at the tests?

[njuettner]

@nenadilic84 I've added a commit to your PR to add the test which covers the cange.
https://coveralls.io/builds/18064739/source?filename=provider/digital_ocean.go#L219

This looks good now 🙂 .

So thanks again for finding this bug 👍 .