client: allow keep-alive for POST requests

client: allow keep-alive for POST requests

Allow POST requests via persistent connections to improve performance
especially when using HTTPS on older devices.

After this change, average page load times in LuCI improve significantly
once the TLS connections are initiated.

When testing an ar71xx 19.07.2 build on an ethernet connected TL-WR1043nd
using luci-ssl-openssl and the ustream-openssl backend, the average page
load time for the main status page decreased to 1.3s compared to 4.7s
before, the interface and wireless configuration pages loaded in 1.2s
seconds each compared to the 4.2s and 4.9s respectively before.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 5e9c23c6f40ff26209ef22cfeeda4904a5918f3d)