HTTP headers 'Host:' and 'User-Agent:' fixed

### Description
The HTTP headers 'Host' and 'User-Agent' were truncated.
That is now fixed and the corresponding functional tests have been modified accordingly.
The version of libcurl has been added to the user-agent.

This PR is related to the issue #442.
Yeah, this define is part of libcurl.
It is a parameter to tell curl_version_info what kind of version to return (or something like that).
Investigate if you are curious, but it works ...

NTC