Set the value to use for the "User-Agent" header on [class@Message]s sent from this.
If user_agent has trailing whitespace, this will append its own product token (eg,
`libsoup/3.0.0`) to the end of the header for you. If user_agent is null then no
"User-Agent" will be included in requests. See [property@Session:user-agent] for more information.
| this |
a Session |
| user_agent |
the user agent string |