Prevent double slashes with HTTP proxy (serenity-rs#2377)

Fixes serenity-rs#2322

Probably wouldn't have caused because double slashes are treated by HTTP
servers as single slashes in my experience.

After a bit of back and forth I opted not to use reqwest methods. Parsing the
given proxy in `HttpBuilder` into an `Url` directly means the builder method
returns `Result` (or panics) which is both ugly. Also, `Url`'s methods yield the
scheme and domain both as `&str`, so we would still just have a string in the
internal proxy field instead of a type-safe value. So I kept the current simple
solution that gets the job done and just added the fix.

src/http/request.rs

@@ -71,7 +71,8 @@ impl<'a> Request<'a> {
         let mut path = self.route.path().to_string();
 
         if let Some(proxy) = proxy {
-            path = path.replace("https://discord.com", proxy);
+            // trim_end_matches to prevent double slashes after the domain
+            path = path.replace("https://discord.com", proxy.trim_end_matches('/'));
         }
 
         if let Some(params) = self.params {