Symptom: Switching between layers with different attributions does
not remove the attribution from the previously selected layer.
Control.Attribution keeps track of its attributions with a counter for
each text. The problem described in #4285 is that each time the layer
is added to the map, the counter is increased by two. It's because
Layer calls Control.Attribution.addAttribution() twice, once from
addLayer() and the second time via the whenReady() callback _layerAdd().
This was not caught by the tests since the callback was never fired
(missing map.setView() call).
Fixes#4285
The rationale is this: the spec string describes the expected
behavior unconditionally. The code examples, on the other hand,
set up an expectation that is tested with the call to the expect
method. The code examples can violate the expectation, but the
spec string does not. The value of the spec string is as clearly
as possible describing the behavior. Including “should” in that
description adds no value. (From http://rubyspec.org/style_guide/)
