provider/aws: Allow updating tuples in WAF SQLInjectionMatchSet + no tuples

[radeksimko]

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

TL;DR We were not deleting any tuples except when deleting the whole Set.

Test plan

make testacc TEST=./
builtin/providers/aws TESTARGS='-run=TestAccAWSWafSqlInjectionMatchSet_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/19 16:07:49 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSWafSqlInjectionMatchSet_ -timeout 120m
=== RUN   TestAccAWSWafSqlInjectionMatchSet_basic
--- PASS: TestAccAWSWafSqlInjectionMatchSet_basic (27.47s)
=== RUN   TestAccAWSWafSqlInjectionMatchSet_changeNameForceNew
--- PASS: TestAccAWSWafSqlInjectionMatchSet_changeNameForceNew (54.99s)
=== RUN   TestAccAWSWafSqlInjectionMatchSet_disappears
--- PASS: TestAccAWSWafSqlInjectionMatchSet_disappears (25.67s)
=== RUN   TestAccAWSWafSqlInjectionMatchSet_changeTuples
--- PASS: TestAccAWSWafSqlInjectionMatchSet_changeTuples (48.09s)
=== RUN   TestAccAWSWafSqlInjectionMatchSet_noTuples
--- PASS: TestAccAWSWafSqlInjectionMatchSet_noTuples (24.95s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	181.207s

cc @yusukegoto

[grubernaut]

[ghost]

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