fix double delegate warning

ruby issues the following warning when using excon:

```
/home/bene/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/forwardable.rb:181: warning: method redefined; discarding old close
/home/bene/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/forwardable.rb:181: warning: previous definition of close was here
```

this patch fixes the double delegation