provider/aws: Allow updating predicates in WAF Rule + no predicates

[radeksimko]

This is to fix a bug very similar to #10403 and #11959

TL;DR We were not deleting any predicates except when deleting the whole Rule.

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSWafRule_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/29 07:22:01 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSWafRule_ -timeout 120m
=== RUN   TestAccAWSWafRule_basic
--- PASS: TestAccAWSWafRule_basic (31.54s)
=== RUN   TestAccAWSWafRule_changeNameForceNew
--- PASS: TestAccAWSWafRule_changeNameForceNew (62.36s)
=== RUN   TestAccAWSWafRule_disappears
--- PASS: TestAccAWSWafRule_disappears (30.40s)
=== RUN   TestAccAWSWafRule_changePredicates
--- PASS: TestAccAWSWafRule_changePredicates (54.65s)
=== RUN   TestAccAWSWafRule_noPredicates
--- PASS: TestAccAWSWafRule_noPredicates (21.86s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	200.836s

cc @yusukegoto

[paddycarver]

Tests pass, looks good to me. I think @radeksimko wanted to get this in before the release, and he's offline, so I'm going to go ahead and merge it and update the CHANGELOG for him.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.