Chain operator should respect array constructor

[netomi]

This PR fixes the problem where when using a HOF like map with a chain operator, the array constructor is not respected which is added to enforce that the output is always an array regardless of the input.

Take this example:

$map([1], function($v){$v*$v})[]

returns correctly [1]. However, when using a chain operator like that:

$ ~> $map(function($v){$v*$v})[]

and providing [1] as input, the singleton value is returned.
This PR fixes that and adds 2 test cases to the test suite.

[mattbaileyuk]

Thanks for fix!