HTTP Keep-Alive header should be respected with connection pooling

[omus]

The HTTP Keep-Alive header includes a timeout parameter which is:

An integer that is the time in seconds that the host will allow an idle connection to remain open before it is closed. A connection is idle if no data is sent or received by a host. A host may keep an idle connection open for longer than timeout seconds, but the host should attempt to retain a connection for at least timeout seconds.

HTTP.jl's idle_timeout connection parameter should probably be updated to respect the timeout parameter specified by the server. This may help with solving problems such as #1141.