CURLOPT_USERAGENT ignored when using CURL_IMPERSONATE env var

[lwthiker]

Reported originally by @momala454 in #42 :

i have a small problem with your pull request (actually, with the whole libcurl version). When i'm using CURL_IMPERSONATE=chrome98, i'm unable to overwrite the default user-agent header to Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.75 Safari/537.36
it stays with Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/98.0.4758.102 Safari/537.36

when I set CURLOPT_USERAGENT, it does not overwrite the default user-agent set with "CURL_IMPERSONATE" env variable. It only reset it if I modify the header "user-agent" using CURLOPT_HTTPHEADER

[lwthiker]

Fixed, using CURLOPT_USERAGENT while the CURL_IMPERSONATE env var is set will now work as expected