Tweak the UNIVERSAL::can override to allow SUPER::foo to work

[TvdW]

SUPER:: is always sort-of broken:

* $obj->SUPER::foo will never actually use $obj for the method
  resolution -- it will use the *package in which the code was
  compiled in*.
* $obj->can('SUPER::foo'), similarly, will use the "current"
  place in the callstack to find the next one.

The problem here is that Class::Std overrides UNIVERSAL::can,
so when it calls $original_can->($class, $method) and $method
is SUPER::foo, the SUPER code will look at the 'current' place
in the callstack.. which is the UNIVERSAL::can override in
Class::Std.

Whoops.

In particular, this was breaking AnyEvent, which uses this
lovely construct:

goto &{ UNIVERSAL::can AnyEvent => "SUPER::$name" }

Previously, SUPER::$name broke if Class::Std was loaded.

[cpansprout]

What if something tries to call UNIVERSAL::can($thing, "SUPER::isa")? With this patch it will return &UNIVERSAL::isa (which Class::Std inherits), instead of whatever other &isa might be in the inheritance chain.

Wouldn’t it be better to goto &$real_can if the method name begins with SUPER::?