IO#reopen will be able to decide an access mode even if it was not pa…

…ssed.

Test Script:
{{{
require 'test/unit/assertions.rb'
include Test::Unit::Assertions

File.delete("/tmp/foo") if File.exists?("/tmp/foo")
File.delete("/tmp/bar") if File.exists?("/tmp/bar")

f = File.open("/tmp/foo", "w")
f.reopen("/tmp/bar")
f.puts "hello, world"
f.close

File.open("/tmp/bar") {|f|
  assert_equal("hello, world\n", f.gets)
}

puts :ok
}}}

io.c

@@ -2794,15 +2794,14 @@ rb_io_reopen(VALUE io, SEL sel, int argc, VALUE *argv)
     rb_io_t *io_s = ExtractIOStruct(io);
 
     // Reassociate it with the stream opened on the given path
-    if (NIL_P(mode_string)) {
-	mode_string = (VALUE)CFSTR("r");
+    if (!NIL_P(mode_string)) {
+	io_s->mode = convert_mode_string_to_fmode(mode_string);
     }
     FilePathValue(path_or_io); // Sanitize the name
     const char *filepath = RSTRING_PTR(path_or_io);
     const int fd =
-	open(filepath, convert_mode_string_to_oflags(mode_string), 0644);
-    prepare_io_from_fd(io_s, fd,
-		       convert_mode_string_to_fmode(mode_string));
+	open(filepath, convert_fmode_to_oflags(io_s->mode), 0644);
+    prepare_io_from_fd(io_s, fd, io_s->mode);
     GC_WB(&io_s->path, path_or_io);
     io_s->buf = NULL;
     io_s->buf_offset = 0;