Placing conditions on optionally associated objects raises exceptions at rule match time

[thatothermitch]

Consider the classes:

class Foo
   attr_accessor :my_bar
end

class Bar
   def baz; "quux"; end
end

The ability:

 can :manage, Foo, :bar => { :baz => "quux" }

And the check:

foo_without_bar = Foo.new
foo_without_bar.bar #  => nil
can? :manage, foo_without_bar # => Exception!

In 1.6.4 this results in the exception undefined method 'bar' for nil:NilClass. This patch modifies the Rule#matches_conditions_hash? to return false for this case.

Test included, and comments are welcome. Thanks!

[ryanb]

Looks great thanks. I'll get this pulled in soon.