provider/fastly: Update Gzip handling with new go-fastly

[catsby]

sethvargo/go-fastly now handles the special case of ContentTypes and Extensions, regarding omitempty, because the Fastly API expects empty values if there are no content types or extensions...

This PR updates the vendored go-fastly and updates the code to remove the blemish.

$ make testacc TEST=./builtin/providers/fastly TESTARGS="-run=TestAccFastlyServiceV1_gzips_basic"
==> Checking that code complies with gofmt requirements...
/Users/clint/Projects/Go/bin/stringer
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/fastly -v -run=TestAccFastlyServiceV1_gzips_basic -timeout 120m
=== RUN   TestAccFastlyServiceV1_gzips_basic
--- PASS: TestAccFastlyServiceV1_gzips_basic (26.15s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/fastly 26.163s

[phinze]

LGTM

[ghost]

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