Make mapping optional for PUT Mapping API request

Put mapping now supports either of these formats:

POST foo/doc/_mapping
{
  "doc": {
    "_routing": {"required": true},
    "properties": {
      "body": {"type": "string"}
    }
  }
}

or

POST foo/doc/_mapping
{
  "_routing": {"required": true},
  "properties": {
    "body": {"type": "string"}
  }
}