Sending a largish POST request with --limit-rate delays receiving of small server response

[Fabian Keil via curl-library]

Proposed fix: https://github.com/curl/curl/pull/6797

Thanks for the update pushed to bagder/http-post-speed-limit.
With the commit 9334e4de31a added, one of my tests for Privoxy
reliably triggers a DEBUGASSERT in curl.

With commit e7dcf9a105 added the test no longer results in
a core dump but it results in a "protocol FAILED!" message.

With a8dfd8bee0 added the test works again and the rest
of my Privoxy tests continue to work.

Thanks.

Fabian