Fixing a small logic bug in diffList

It’s not enough to only check if no new value is set. It can also be
that a new value is set, but contains a variable that cannot be
interpolated until a depending resource is created during the apply
fase.

I actually found this one as one of the acceptance tests for the AWS
ELB resource was failing. It failed with the following error:

```
--- FAIL: TestAccAWSELB_InstanceAttaching (177.83 seconds)
  testing.go:121: Step 1 error: Error applying: aws_elb.bar: diffs
didn't match during apply. This is a bug with the resource provider,
please report a bug.
FAIL
exit status 1
FAIL  github.com/hashicorp/terraform/builtin/providers/aws  177.882s
```

After a quick look I noticed it was actually a bug in core TF so added
the test and made sure all unit tests and AWS acceptance tests are now
running successfully.
Merge pull request #661 from svanharmelen/f-fix-difflist-logic-error

core: fixing a small logic bug in diffList