Infinite loop on notifying with self-referential context

Sent this in an email as well but just thought I'd open an issue for it - if you try to notify with a context that refers to itself then the bugsnag gem goes into an infinite loop:
```
Loading development environment (Rails 3.2.13)
1.9.1 :001 > context = {}
 => {}
1.9.1 :002 > context[:key] = context
 => {:key=>{...}}
1.9.1 :003 > Bugsnag.notify(StandardError.new, context)
SystemStackError: stack level too deep
        from /home/arkadiy/.rvm/rubies/ruby-1.9.3-p448/lib/ruby/1.9.1/irb/workspace.rb:80
Maybe IRB bug!
1.9.1 :004 >
```