Kibana version: 7.8.1

Elasticsearch version: 7.8.1

Server OS version: Ubuntu 16.04.6 LTS

Browser version: Firefox 79.0b9

Browser OS version: macOS Catalina 10.15.6

Original install method (e.g. download page, yum, from source, etc.): apt

Describe the bug: Some responses from /internal/search/es respond with empty status code, header, and body. Presumably from cancelled queries.

Steps to reproduce:

1. Load up a dashboard and query/filter
2. Remove filters
3. Re-add filters
4. Observe network tab with blank status codes, zero headers, and no response body.

Expected behavior:
Network log should contain proper queries to the ES API with corresponding responses. Cancelled queries should respond with a proper HTTP status code (204 seems appropriate).

Screenshots (if relevant):

Errors in browser console (if relevant):
Failed to load resource: the server responded with a status of 502 (Proxy Error) - this only occurs when Kibana is reverse proxied.

Any additional context:
This is primarily an issue when reverse proxying Kibana. Apache fusses when it receives an upstream response that doesn't have a valid header or is missing the response code.

A temporary remediation for this with Apache is setting ProxyBadHeader Ignore in the server config. Obviously this is not the ideal solution, since the cancelled inflight requests should return a valid HTTP status code despite responding with no content.

Edit: The ProxyBadHeader fix doesn't fully resolve the problem. Not 100% why but I speculate that the upstream server is both sending empty responses and terminating requests early.