Prevent a general 'user-agent' setting, i.e. extractor.user-agent, from
overriding the User-Agent header set by an extractor's 'browser'
option, default or otherwise, meaning only extractor-level or top-level
'user-agent' settings will override the 'browser' User-Agent.
https://github.com/mikf/gallery-dl/issues/7382#issuecomment-2985296321