Log Message:
cpp-httplib: update to 0.18.3.

0.18.3

Regression: Client keep-alive subsequent requests very slow #1997
304 Not Modified response stalls until timeout #1998

0.18.2

Fix the problem that CreateFile2 in mmap::open fails to … #1973
Default Accept-Encoding header for the client #1975
SSLClientReconnection fails on Windows #1980
delay in keep_alive due to sleep #1969
missing query pararm in httplib::Client::send #1985