provider/aws: Allow updating tuples in WAF ByteMatchSet + 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 ByteMatchSet.

While fixing IPSet I noticed all the other WAF resources are affected too, so this is second PR fixing another resource - more will follow.

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSWafByteMatchSet_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/28 11:49:01 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSWafByteMatchSet_ -timeout 120m
=== RUN   TestAccAWSWafByteMatchSet_basic
--- PASS: TestAccAWSWafByteMatchSet_basic (22.73s)
=== RUN   TestAccAWSWafByteMatchSet_changeNameForceNew
--- PASS: TestAccAWSWafByteMatchSet_changeNameForceNew (44.36s)
=== RUN   TestAccAWSWafByteMatchSet_changeTuples
--- PASS: TestAccAWSWafByteMatchSet_changeTuples (40.43s)
=== RUN   TestAccAWSWafByteMatchSet_noTuples
--- PASS: TestAccAWSWafByteMatchSet_noTuples (21.91s)
=== RUN   TestAccAWSWafByteMatchSet_disappears
--- PASS: TestAccAWSWafByteMatchSet_disappears (19.47s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	148.933s

cc @yusukegoto

[catsby]

LGTM 👍

[ghost]

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