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.
This has been removed due to now you manage these using char[] of fixed length? Just to ensure we don't introduce a new leak... ;)