helper/schema: fix validating nested objects

When interpreting a nested object, we were validating against the "raw"
value, and not the interpolated value, causing incorrect errors.

This affects structures such as:

```terraform
tags = "${list(map("foo", "bar"))}"
```

Prior to this, a complaint about "expected object, got string" since the
raw value is obviously a string, when the interpolated value is the
correct shape.

helper/schema/schema.go

@@ -1373,7 +1373,7 @@ func (m schemaMap) validateObject(
 	k string,
 	schema map[string]*Schema,
 	c *terraform.ResourceConfig) ([]string, []error) {
-	raw, _ := c.GetRaw(k)
+	raw, _ := c.Get(k)
 	if _, ok := raw.(map[string]interface{}); !ok {
 		return nil, []error{fmt.Errorf(
 			"%s: expected object, got %s",