test_io.rb: test for write

* test/ruby/test_io.rb (TestIO#test_write_32bit_boundary): add test
  for write part.  [ruby-core:55098] [Bug #8431]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40894 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-05-23

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40895 b2dd03c8-39d4-4d8f-98ff-823fe69b080e