rumonade breaks flatten on arrays containing a hash (fix)

The following error occurs when rumonade is around:
```
2.0.0p195 :001 > require 'rumonade'
 => true 
2.0.0p195 :002 > [ {foo: 1}, 1, 'a' ].flatten
TypeError: no implicit conversion of Array into Hash
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/hash.rb:22:in `merge'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/hash.rb:22:in `block in bind'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/hash.rb:22:in `each'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/hash.rb:22:in `inject'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/hash.rb:22:in `bind'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/monad.rb:62:in `flatten_with_monad'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/monad.rb:62:in `block in flatten_with_monad'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/array.rb:21:in `call'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/array.rb:21:in `block in bind'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/array.rb:21:in `each'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/array.rb:21:in `inject'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/array.rb:21:in `bind'
	from /home/mvielsmaier/.rvm/gems/ruby-2.0.0-p195/gems/rumonade-0.4.2/lib/rumonade/monad.rb:62:in `flatten_with_monad'
	from (irb):2
	from /home/mvielsmaier/.rvm/rubies/ruby-2.0.0-p195/bin/irb:16:in `<main>'
```

without rumonade:
```
2.0.0p195 :001 > [ {foo: 1}, 1, 'a' ].flatten
 => [{:foo=>1}, 1, "a"] 
```

I created a test that demonstrates the problem and fixed it.