This is consistent with 0.5.x and seems more useful. Updating
the state of a layer switcher using map.hasLayer in response
to layeradd and layerremove events is my use case.
Plugins that need to bind events to `window` or `document`
can attach a listener for this event and unbind their event
handlers.
However, for symmetry, unload is fired only if load has been
fired.
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/)
