string.c: enable capacity when setting capa

* string.c (rb_str_modify_expand): enable capacity and disable
  assocation with packed objects when setting capa, so that
  pack("p") string fails to unpack properly after modified.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44803 b2dd03c8-39d4-4d8f-98ff-823fe69b080e