Fix lost support for contents without acquisition

See https://github.com/plone/plone.app.z3cform/commit/587e229e267705a4fd48c6c51a76f849196fceba#commitcomment-2606236

Apart the fix discussed in the commit above, I found that the widget class was not covered by tests.
I added a minimal test for the acquisition stuff but is probably **ugly**, so nuke it away if is not appropriate