attribute method should respect local variable names before attempting to invoke the symbol on current_object

Example

``` ruby
get '/hello' do
  user = User.some_lookup_method(params['id])
  bldr :'template', :locals => {:user => user, :raw_value => rand(10)}
end

# template.bldr
object :user => user do
  attribute :raw_value => raw_value # boom!
end
```

https://github.com/ajsharp/bldr/blob/master/lib/bldr/node.rb#L166
