add comment and unit test

Author: dsherret

url/src/parser.rs

@@ -398,6 +398,8 @@ impl<'a> Parser<'a> {
     }
 
     pub fn parse_scheme<'i>(&mut self, mut input: Input<'i>) -> Result<Input<'i>, ()> {
+        // starts_with will also fail for empty strings
+        // so we can skip that comparison for perf
         if !input.starts_with(ascii_alpha) {
             return Err(());
         }

url/tests/unit.rs

@@ -1031,6 +1031,14 @@ fn test_set_scheme_to_file_with_host() {
     assert_eq!(result, Err(()));
 }
 
+#[test]
+fn test_set_scheme_empty_err() {
+    let mut url: Url = "http://localhost:6767/foo/bar".parse().unwrap();
+    let result = url.set_scheme("");
+    assert_eq!(url.to_string(), "http://localhost:6767/foo/bar");
+    assert_eq!(result, Err(()));
+}
+
 #[test]
 fn no_panic() {
     let mut url = Url::parse("arhttpsps:/.//eom/dae.com/\\\\t\\:").unwrap();