Unable to unset index.blocks.read_only without index. prefix

Elasticsearch version (bin/elasticsearch --version): 7.3.2 / 7.4.0

Description of the problem including expected versus actual behavior:

After creating an index with blocks.read_only set to true, it was nearly impossible to unset the setting (in order to delete or close the index) until it was pointed out to try it with the index. prefix.

Steps to reproduce:

1. Create an index with blocks.read_only set to true

PUT /test
{
  "settings": {
    "blocks.read_only": true
  }
}

2. Attempt to unset it with blocks.read_only set to false or null (or attempt to close or delete the index without setting it, which by design it's blocked, but I am including these to help people find this issue).

PUT /test/_settings
{
  "blocks.read_only": null
}

3. Unset it with index.blocks.read_only set to false or null

PUT /test/_settings
{
  "index.blocks.read_only": null
}