🎨 style: change http/https header word/sentence casing (#555)

Currently all HTTP request headers are in snake case and upper case.
The adopted standard is kebab case and title case.

Here you will find more details:
https://en.wikipedia.org/wiki/List_of_HTTP_header_fields#Standard_request_fields

src/engines/brave.rs

@@ -54,14 +54,14 @@ impl SearchEngine for Brave {
         };
 
         let header_map = HeaderMap::try_from(&HashMap::from([
-            ("USER_AGENT".to_string(), user_agent.to_string()),
+            ("User-Agent".to_string(), user_agent.to_string()),
             (
-                "CONTENT_TYPE".to_string(),
+                "Content-Type".to_string(),
                 "application/x-www-form-urlencoded".to_string(),
             ),
-            ("REFERER".to_string(), "https://google.com/".to_string()),
+            ("Referer".to_string(), "https://google.com/".to_string()),
             (
-                "COOKIE".to_string(),
+                "Cookie".to_string(),
                 format!("safe_search={safe_search_level}"),
             ),
         ]))