Some endpoints return Content-Length: 0 for HEAD requests

[nik9000]

After #21123 when Elasticsearch receive a HEAD request it returns the Content-Length of the that it would return for a GET request with an empty response body. Except in the document exists, index exists, and type exists requests which return 0. We should fix them to also return the Content-Length that would be in the response.

[jasontedor]

I'm adding the v5.1.0 label too, I think we should target a fix there.