Let parse_mode() set the default value

Author: certik

src/stdlib_experimental_io.f90

@@ -297,8 +297,11 @@ integer function open(filename, mode) result(u)
 character(:),allocatable :: action_, position_, status_, access_, form_
 
 
-mode_ = "r t"
-if (present(mode)) mode_ = parse_mode(mode)
+! Note: with `optval` this could be written as:
+! mode_ = parse_mode(optval(mode, ""))
+mode_ = ""
+if (present(mode)) mode_ = mode
+mode_ = parse_mode(mode_)
 
 if (mode_(1:2) == 'r ') then
     action_='read'