Avoid NoMethodError for the enumerable_select_with_index
Correct to work Observable#zip

  require 'rx'
  require 'core_ext/enumerable'

  a = [ 4, 5, 6 ].to_observable
  b = [ 7, 8, 9 ].to_observable

  sub = a.zip(b).subscribe_on_next {|arr| puts arr.to_s }
  sub.unsubscribe
added Observable.from_array for some examples