Value of unchecked checkboxes changed from False to [] in zope.testbrowser 5.0.0

[mgedmin]

The old mechanize-based zope.testbrowser behaved like this:

>>> print(browser.contents)
 ...
<input type="checkbox" id="closed" name="closed" value="on" />
...
>>> browser.getControl(name='closed').value
False

The new webtest-based zope.testbrowser returns [] instead of False. Which is fine, I suppose, but I'd like this backwards incompatibility to be documented somewhere.