Revert readline optimization and add test

src/libcore/io.rs

@@ -185,13 +185,13 @@ impl<T: Reader> T : ReaderUtil {
     }
 
     fn read_line(&self) -> ~str {
-        let mut line = ~"";
+        let mut bytes = ~[];
         loop {
             let ch = self.read_byte();
             if ch == -1 || ch == 10 { break; }
-            str::push_char(&mut line, ch as char);
+            bytes.push(ch as u8);
         }
-        line
+        str::from_bytes(bytes)
     }
 
     fn read_chars(&self, n: uint) -> ~[char] {
@@ -1221,6 +1221,14 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_read_line_utf8() {
+        do io::with_str_reader(~"生锈的汤匙切肉汤hello生锈的汤匙切肉汤") |inp| {
+            let line = inp.read_line();
+            assert line == ~"生锈的汤匙切肉汤hello生锈的汤匙切肉汤";
+        }
+    }
+
     #[test]
     fn test_readchars_wide() {
         let wide_test = ~"生锈的汤匙切肉汤hello生锈的汤匙切肉汤";