bool not string
Fixes: #11010
Adds a test to show cover the use-case that the OP suggested caused the
panic
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDataSourceIAMServerCertificate_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/01/03 22:39:21 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDataSourceIAMServerCertificate_ -timeout 120m
=== RUN TestAccAWSDataSourceIAMServerCertificate_basic
--- PASS: TestAccAWSDataSourceIAMServerCertificate_basic (19.48s)
=== RUN TestAccAWSDataSourceIAMServerCertificate_matchNamePrefix
--- PASS: TestAccAWSDataSourceIAMServerCertificate_matchNamePrefix (1.95s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 21.454s
```
56c352a535