HTTP Platform-Dependent Client Wrapper.

I suppose our client should not download server reply into the file in any case except of HTTP::200.
Usually servers or in-between proxies return some random html page for 4XX errors which was definitely not expected by user of our code, huh?
Later we can probably offer more flexible solution for those who want to parse such stuff (or we can alternatively save such bodies now in memory instead of writing them into the file).