Fix compatibity with RSpec 2.12

This fixes several test failures with RSpec 2.12, such as bellow:
```
  1) SimpleNavigation::Helpers active_navigation_item active item_container for desired level exists container has selected item 
     Failure/Error: it {@controller.active_navigation_item(:level => 2).should == @subnav1_item}
     ArgumentError:
       wrong number of arguments (1 for 0)
     # /usr/share/gems/gems/rspec-expectations-2.12.1/lib/rspec/matchers/operator_matcher.rb:67:in `uses_generic_implementation_of?'
     # /usr/share/gems/gems/rspec-expectations-2.12.1/lib/rspec/matchers/operator_matcher.rb:34:in `block in use_custom_matcher_or_delegate'
     # /builddir/build/BUILD/rubygem-simple-navigation-3.10.0/usr/share/gems/gems/simple-navigation-3.10.0/spec/lib/simple_navigation/rendering/helpers_spec.rb:92:in `block (5 levels) in <top (required)>'
```