Fix OrderOperation::count()

[mdumoulin]

Reason for this PR

The OrderOperation::count() method no longer works as expected.

Steps to reproduce

• Given I add operations to OrderOperation :

$operation = new OrderOperation();
$operation
    ->accept('ORDER123', 'ChannelA')
    ->cancel('ORDER456', 'ChannelB')
    ->ship('ORDER789', 'ChannelC');

• When I count operations

$operation->count()

Observed bug on master

• Then I get 0

Expected result on branch

• Then I get 3