Cannot work some examples

I have fixed core_ext/enumerable.rb to work the below script.

```ruby
$: << './lib'

require 'rx'
require 'core_ext/enumerable.rb'

sub = [4, 5, 6].to_observable
  .select {|x| x.even? }
  .map {|x| x * x }
  .subscribe_on_next {|x| puts x.to_s }
```