The bug affects write operations at least when Tempfile.create is
used in text mode. This patch aims to make IO#write consistent with
MRI's io_write.

I saw no test failing because of the patch. Neither could I find an obvious reason for str being dup'ed or frozen.

The bug is only present on master:

$ bin/jruby -e "require 'tempfile'; Tempfile.create('test', textmode: true) { |f| f.write 'x' }"
RuntimeError: can't modify frozen string
               write at org/jruby/RubyIO.java:1429
               write at org/jruby/RubyIO.java:1399