Escape control characters when encoding strings

Closes #126

src/display.rs

@@ -49,6 +49,9 @@ fn write_str(f: &mut fmt::Formatter, s: &str) -> fmt::Result {
             '\u{d}' => try!(write!(f, "\\r")),
             '\u{22}' => try!(write!(f, "\\\"")),
             '\u{5c}' => try!(write!(f, "\\\\")),
+            c if c < '\u{1f}' => {
+                try!(write!(f, "\\u{:04}", ch as u32))
+            }
             ch => try!(write!(f, "{}", ch)),
         }
     }

src/lib.rs

@@ -489,4 +489,10 @@ mod tests {
         assert_eq!(Value::Integer(0), *value.lookup("table.\"element\".\"value\".0").unwrap());
     }
 
+    #[test]
+    fn control_characters() {
+        let value = Value::String("\x05".to_string());
+        assert_eq!(value.to_string(), r#""\u0005""#);
+    }
+
 }