helper/schema: Support serialization of bool values in TypeMap values [v1]

[bflad]

Reference: hashicorp/terraform-provider-aws#13549

This may only be an issue with Terraform 0.11 and earlier with an obscure schema of:

"attr": {
	Type:     schema.TypeSet,
	Optional: true,
	Elem: &schema.Schema{
		Type: schema.TypeMap,
		Elem: &schema.Schema{
			Type: schema.TypeString,
		},
	},
},

And having the following Terraform configuration:

resource "example_thing" "example" {
  attr = [{
    somekey = true
  }]
}

Previously, this would panic (with updated test):

$ go test ./helper/schema -run=TestSerializeForHash
--- FAIL: TestSerializeForHash (0.00s)
panic: unknown value type in TypeMap bool [recovered]
  panic: unknown value type in TypeMap bool

goroutine 5 [running]:
testing.tRunner.func1.1(0x1a4a5e0, 0xc0004b9920)
  /usr/local/Cellar/go/1.14.3/libexec/src/testing/testing.go:940 +0x2f5
testing.tRunner.func1(0xc000372c60)
  /usr/local/Cellar/go/1.14.3/libexec/src/testing/testing.go:943 +0x3f9
panic(0x1a4a5e0, 0xc0004b9920)
  /usr/local/Cellar/go/1.14.3/libexec/src/runtime/panic.go:969 +0x166
github.com/hashicorp/terraform-plugin-sdk/helper/schema.SerializeValueForHash(0xc000085bb8, 0x1aaef40, 0xc000528450, 0xc000531400)
  /Users/bflad/src/github.com/hashicorp/terraform-plugin-sdk/helper/schema/serialize.go:61 +0x8c9
github.com/hashicorp/terraform-plugin-sdk/helper/schema.TestSerializeForHash(0xc000372c60)
  /Users/bflad/src/github.com/hashicorp/terraform-plugin-sdk/helper/schema/serialize_test.go:230 +0x1520
testing.tRunner(0xc000372c60, 0x1c2e848)
  /usr/local/Cellar/go/1.14.3/libexec/src/testing/testing.go:991 +0xdc
created by testing.(*T).Run
  /usr/local/Cellar/go/1.14.3/libexec/src/testing/testing.go:1042 +0x357
FAIL  github.com/hashicorp/terraform-plugin-sdk/helper/schema 5.797s

[appilon]

I'm going to suggest porting this codechange forward to V2 even if it's not a bug in 0.12

[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.

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