test_member.rb: fix locale depending test failure

* test/-ext-/struct/test_member.rb (test_member_get): set
  default_external encoding to fix locale depending test failure.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45466 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-03-29

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45467 b2dd03c8-39d4-4d8f-98ff-823fe69b080e