merge revision(s) 44803: [Backport #9478]

	* 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/branches/ruby_2_1@44819 b2dd03c8-39d4-4d8f-98ff-823fe69b080e