Mark array as "going to be modified" in `Array#reject!`

	Before this patch, if `reject!` is called on a shared array it can
	mutate the shared array rather than a copy.  This patch marks the array
	as "going to be modified" so that the shared source array isn't
	mutated.

	[Bug #15479] [ruby-core:90781]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66847 b2dd03c8-39d4-4d8f-98ff-823fe69b080e