The HTTP User-Agent "ansible-httpget" is already kind of the default,
it being the default value provided by the `url_argument_spec` helper
method. Yet, it may not be practical for all modules to get their
argument_spec that way.
Without a default User-Agent we fall back on the library
User-Agent. That being something like "Python-urllib/2.7".
While I'm no big fan of web servers making decisions based on the
provided User-Agent I still think that part of being a well-behaved
HTTP client is to provide an informative User-Agent. Not to mention
that it's a good thing for Ansible to behave consistently.
Indirectly fixes#26239
063f5d0ca8