merge revision(s) ec8e5f5a,5a187e26: [Backport #15952]

	array.c: always check frozenness in Array#unshift. Fixes [Bug #15952]

	Closes: https://github.com/ruby/ruby/pull/2251

	array.c add back shared array optimization to
	 ary_ensure_room_for_unshift

	Bug fix in commit ec8e5f5a [Bug #15952] disabled an
	optimization in this function.

	Closes: https://github.com/ruby/ruby/pull/2252

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67740 b2dd03c8-39d4-4d8f-98ff-823fe69b080e