Clippy: Fixed some clippy warnings (#31818)

* Fixed clippy warnings

* made changes for lowercase characters.

* changed is_lowercase() to is_ascii_lowercase()

* added std library function `is_ascii_uppercase()` and `is_ascii_lowercase()`

* made recommended changes

components/script/dom/headers.rs

@@ -567,5 +567,5 @@ pub fn extract_mime_type(headers: &HyperHeaders) -> Option<Vec<u8>> {
     }
 
     // Step 7, 8
-    return mime_type.map(|m| format!("{}", m).into_bytes());
+    mime_type.map(|m| format!("{}", m).into_bytes())
 }