TextArea: fix off-by-one required_size

examples/text_area.rs

@@ -11,8 +11,7 @@ fn main() {
         .title("Describe your issue")
         .padding((1, 1, 1, 0))
         .content(TextArea::new()
-            .with_id("text")
-            .fixed_size((30, 5)))
+            .with_id("text"))
         .button("Ok", Cursive::quit));
 
     siv.run();

src/views/text_area.rs

@@ -366,7 +366,7 @@ impl View for TextArea {
     fn required_size(&mut self, constraint: Vec2) -> Vec2 {
         self.compute_rows(constraint);
         Vec2::new(
-            self.rows.iter().map(|r| r.width).max().unwrap_or(1),
+            1 + self.rows.iter().map(|r| r.width).max().unwrap_or(1),
             self.rows.len()
         )
     }