Fix string octal escape encoding

The bug affected invisible ASCII characters that have one of the two high bits set, so only DEL.

Author: laurmaedje

src/buf.rs

@@ -76,7 +76,7 @@ impl BufExt for Vec<u8> {
         }
 
         self.push(octal(value >> 6));
-        self.push(octal((value >> 3) & 63));
+        self.push(octal((value >> 3) & 7));
         self.push(octal(value & 7));
     }
 }

src/object.rs

@@ -1088,6 +1088,7 @@ mod tests {
         test_primitive!(Str(br"\n"), br"(\\n)");
         test_primitive!(Str(b"a\x14b"), br"(a\024b)");
         test_primitive!(Str(b"\xFF\xAA"), b"<FFAA>");
+        test_primitive!(Str(b"\x0A\x7F\x1F"), br"(\n\177\037)");
 
         // Test text strings.
         test_primitive!(TextStr("Hallo"), b"(Hallo)");