Ensure partition-exports kind gets marshalled

The api module has decoding functions that rely on 'kind' being present
of payloads. This is so that we can decode into the appropriate api type
for the config entry.

This commit ensures that a static kind is marshalled in responses from
Consul's api endpoints so that the api module can decode them.

agent/structs/config_entry_exports.go

@@ -1,6 +1,7 @@
 package structs
 
 import (
+	"encoding/json"
 	"fmt"
 
 	"github.com/hashicorp/consul/acl"
@@ -158,3 +159,19 @@ func (e *PartitionExportsConfigEntry) GetEnterpriseMeta() *EnterpriseMeta {
 
 	return &e.EnterpriseMeta
 }
+
+// MarshalJSON adds the Kind field so that the JSON can be decoded back into the
+// correct type.
+// This method is implemented on the structs type (as apposed to the api type)
+// because that is what the API currently uses to return a response.
+func (e *PartitionExportsConfigEntry) MarshalJSON() ([]byte, error) {
+	type Alias PartitionExportsConfigEntry
+	source := &struct {
+		Kind string
+		*Alias
+	}{
+		Kind:  PartitionExports,
+		Alias: (*Alias)(e),
+	}
+	return json.Marshal(source)
+}