CurlFile - update 02

This is second part of larger PR #3226 (first part: PR #3462).

In this PR I add ability to request some specific charset from HTTP-server, set default to "UTF-8,*;q=0.8" (means "I want UTF-8, if available, but I'll will accept any other if UTF-8 not available").

And removed explicit addition of "Connection: keep-alive" as cURL lib will do it already (to prevent duplication of request header parameters).