mget API should support global routing parameter

mget API support `_routing` field but not `routing` parameter.

Reproduction here:

```sh
curl -XDELETE "http://localhost:9200/test/"; echo
curl -XPUT "http://localhost:9200/test/" -d'{
   "settings": {
      "number_of_replicas": 0,
      "number_of_shards": 5
   }
}'; echo

curl -XPUT 'http://localhost:9200/test/order/1-1?routing=key1' -d '{
   "productName":"doc 1"
}'; echo
curl -XPUT 'http://localhost:9200/test/order/1-2?routing=key1' -d '{
   "productName":"doc 2"
}'; echo
curl -XPUT 'http://localhost:9200/test/order/1-3?routing=key1&refresh=true' -d '{
   "productName":"doc 3"
}'; echo

curl -XPOST 'http://localhost:9200/test/order/_mget?pretty' -d '{
    "docs" : [
        {
            "_index" : "test",
            "_type" : "order",
            "_id" : "1-1",
            "_routing" : "key1"
        },
        {
            "_index" : "test",
            "_type" : "order",
            "_id" : "1-2",
            "_routing" : "key1"
        },
        {
            "_index" : "test",
            "_type" : "order",
            "_id" : "1-3",
            "_routing" : "key1"
        }
    ]
}'; echo

curl -XPOST 'http://localhost:9200/test/order/_mget?pretty&routing=key1' -d '{
	"ids": [
		"1-1",
		"1-2",
		"1-3"
	]
}'; echo
```

Closes #3996.