RuntimeError "invalid binding to detach" in EM::Client::close

Error happens when you close EM::Client with no query running.

```
RuntimeError invalid binding to detach
.../ruby-2.0.0-p353/gems/eventmachine-1.0.3/lib/em/connection.rb:272:in `detach_fd'
.../ruby-2.0.0-p353/gems/eventmachine-1.0.3/lib/em/connection.rb:272:in `detach'
.../ruby-2.0.0-p353/gems/mysql2-0.3.14/lib/mysql2/em.rb:29:in `close'
```