When each() takes arguments, it is never safe to assume that the iteration
would repeat the same number of times as with each() without any
argument.  Actually, there is no way to get the exact number, so the
size should be set to nil to denote that.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65302 b2dd03c8-39d4-4d8f-98ff-823fe69b080e