fix: use lower case domain string when using resolve and resolve_to_addrs (seanmonstar#2235)

Author: Alvenix

src/async_impl/client.rs

@@ -1754,7 +1754,7 @@ impl ClientBuilder {
     pub fn resolve_to_addrs(mut self, domain: &str, addrs: &[SocketAddr]) -> ClientBuilder {
         self.config
             .dns_overrides
-            .insert(domain.to_string(), addrs.to_vec());
+            .insert(domain.to_ascii_lowercase(), addrs.to_vec());
         self
     }