Add default combining behavior to combine_latest operator

[schmich]

When no block is specified, the combine_latest operator should combine its two streams into an array (a pair of elements).

Currently, combine_latest errors when no block is specified:

require 'rx'
require 'pp'

left = Rx::Subject.new
right = Rx::Subject.new

lo = left.as_observable
ro = right.as_observable

lo.combine_latest(ro).subscribe { |x| pp x }

left.on_next(1);
right.on_next(1);
left.on_next(2);
right.on_next(2);

# Error output (truncated):
# rx-0.0.3/lib/rx/operators/multiple.rb:168:in `block (3 levels) in combine_latest': undefined method `call' for nil:NilClass (NoMethodError)

In the RxJS implementation, elements from both streams are combined into an array by default:

let left = new Rx.Subject();
let right = new Rx.Subject();

let lo = left.asObservable();
let ro = right.asObservable();

lo.combineLatest(ro).subscribe(x => console.log(x));

left.onNext(1);
right.onNext(1);
left.onNext(2);
right.onNext(2);

/*
As expected, this prints:
[1, 1]
[2, 1]
[2, 2]
*/

In RxRuby, this is also the behavior with the static version of combine_latest, i.e. Rx::Observable.combine_latest(lo, ro).subscribe { |x| pp x } works as expected (see implementation here)

This patch updates the instance version of combine_latest to match the static version and makes lo.combine_latest(ro).subscribe { |x| pp x } work as expected by combining the elements into an array.