Fix .max_line() when source has a trailing newline

Fixes a crash when testing #90 against trunk.

src/lines.rs

@@ -167,7 +167,7 @@ pub trait MaxLine {
 
 impl<S: AsRef<str>> MaxLine for S {
     fn max_line(&self) -> LineNumber {
-        (max(1, self.as_ref().lines().count()) - 1).into()
+        (max(1, self.as_ref().split('\n').count()) - 1).into()
     }
 }
 
@@ -238,6 +238,12 @@ mod tests {
         assert_eq!(line.max_line().0, 0);
     }
 
+    #[test]
+    fn str_max_line_trailing_newline() {
+        let line: String = "foo\nbar\n".into();
+        assert_eq!(line.max_line().0, 2);
+    }
+
     #[test]
     fn from_offsets_relative_to() {
         let newline_positions: NewlinePositions = "foo\nbar".into();