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.
Is there any limit in the HTTP RFC (or any other related standard) for Host header?

Maybe we should use MAX_HOST_HEADER_LENGTH or something like that... to ease changing it if we find it is not enough.