RSpec doesn't work at ruby 1.9.3 because Enumerable#reject will return Array instance only

Created by: sorah

rspec won't run at next release, because at r30148 and #4136 of ruby.

% ruby -ve "class Foo < Array;end; p Foo.new(0,1).reject(&:zero?).class"
ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-darwin10.4.0]
Foo

but at ruby-trunk,

% ruby -ve "class Foo < Array;end; p Foo.new(0,1).reject(&:zero?).class"
ruby 1.9.3dev (2010-12-22 trunk 30290) [x86_64-darwin10.4.0]
Array

RSpec works well after merge this patch.