test_marshal.rb: fix test depending on heap address

* test/ruby/test_marshal.rb (test_packed_string): explicitly force
  encoding for the case packed string is 7bit ASCII only.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44854 b2dd03c8-39d4-4d8f-98ff-823fe69b080e