disable ability to make a doc named /test/test/_query

[erickt]

I was playing around with delete-by-query, and I found it's possible to make a document with the id _query:

% curl -XPOST "http://localhost:9200/test/test/_query" -d "{}"
{"ok":true,"_index":"test","_type":"test","_id":"_query","_version":1}
% curl -XGET "http://localhost:9200/test/test/_query"
{"_index":"test","_type":"test","_id":"_query","_version":1,"exists":true, "_source" : {}}

Could this be made an error? The only way to get rid of this document is to use delete-by-query:

% curl -XDELETE "http://localhost:9200/test/test/_query"
{"error":null}
% curl -XDELETE "http://localhost:9200/test/test/_query" -d '{"ids":{"values":["_query"]}}'
{"ok":true,"_indices":{"test":{"_shards":{"total":5,"successful":5,"failed":0}}}}
% curl -XGET "http://localhost:9200/test/test/_query"
{"_index":"test","_type":"test","_id":"_query","exists":false}

[clintongormley]

Closing in favour of #6736