Fix Infection config builder: make sure it always creates an array for exluded dirs but not object

[maks-rafalko]

Fixes #580

array_unique([['a', 'a', 'b']) returns [0 => 'a', 2 => 'b'] which in its turn breaks infection.json because now we have an object

{
    "excludeDirs": {
        "0": "a",
        "2": "b",
    }
}

instead of array

{
    "excludeDirs": [
        "a",
        "b",
    ]
}

[BackEndTea]

👍

Maybe this could target 0.13? as it is bugfix and not a new feature

[sidz]

👍

[localheinz]

👍

[maks-rafalko]

Maybe this could target 0.13? as it is bugfix and not a new feature

retargeted

Thank you @BackEndTea, @sidz and @localheinz