Unit tests to widgets\ActiveField and refactoring.

Feel free to review, discuss and even cherry-pick.

I would also call for your attention to the test ``testRenderWithCallableContent()``, the ``$expectedValue``.
Shouldn't the field be wrapped only around the callable anonymous function, without a call to the ``begin()`` and ``end()`` methods when content it's callable?

Refactor applied - Method extraction, Compose method and Accumulation/Collecting parameter.