Fix and associated test for composite keys causing type errors

Composite keys with a given key, then a serial for uniqueness, were causing the following type error on update (amongst other things):

```ruby
TypeError: no implicit conversion of Fixnum into String
	from /Users/cknight/.rvm/gems/ruby-2.0.0-p247/gems/dm-redis-adapter-0.8.3/lib/dm-redis-adapter/adapter.rb:153:in `+'
	from /Users/cknight/.rvm/gems/ruby-2.0.0-p247/gems/dm-redis-adapter-0.8.3/lib/dm-redis-adapter/adapter.rb:153:in `block in key_query'
	from /Users/cknight/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/set.rb:232:in `each_key'
	from /Users/cknight/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/set.rb:232:in `each'
	from /Users/cknight/.rvm/gems/ruby-2.0.0-p247/gems/dm-redis-adapter-0.8.3/lib/dm-redis-adapter/adapter.rb:152:in `key_query'
	from /Users/cknight/.rvm/gems/ruby-2.0.0-p247/gems/dm-redis-adapter-0.8.3/lib/dm-redis-adapter/adapter.rb:182:in `records_for'
	from /Users/cknight/.rvm/gems/ruby-2.0.0-p247/gems/dm-redis-adapter-0.8.3/lib/dm-redis-adapter/adapter.rb:77:in `update'
	from /Users/cknight/.rvm/gems/ruby-2.0.0-p247/gems/dm-core-1.2.1/lib/dm-core/repository.rb:180:in `update'
```

A test has been added to cover the fix. Revert the fix to see the test fail.

All original tests still pass, so hopefully things are all still fine...!