Fix up the MapWalk function so that it properly handles nested map[interface{}]interface{}

[mkeeler]

Fixes #5746

mkeeler-corp:~ mkeeler$ cat proxy.json
{
   "Kind": "proxy-defaults",
   "Name": "global",
   "Config": {
      "nested": {
         "foo": "bar"
      }
   }
}
mkeeler-corp:~ mkeeler$ curl -XPUT localhost:8500/v1/config -d @proxy.json
true
mkeeler-corp:~ mkeeler$ consul config read -kind proxy-defaults -name global
{
    "Kind": "proxy-defaults",
    "Name": "global",
    "Config": {
        "nested": {
            "foo": "bar"
        }
    },
    "CreateIndex": 11,
    "ModifyIndex": 23
}

This fixes the JSON serialization errors with map[interface{}]interface{}