Beefcake::Message::RequiredFieldNotSetError: Field key is required but nil

I want a map with a Riak-assigned name. However, I get an exception message.

```
$ pry
[1] pry(main)> require 'riak'
=> true
[2] pry(main)> client = Riak::Client.new(host: '127.0.0.1', pb_port: 17017)
=> #<Riak::Client [#<Node 127.0.0.1:17017>]>
[3] pry(main)> map = Riak::Crdt::Map.new client.bucket('doc'), nil
Beefcake::Message::RequiredFieldNotSetError: Field key is required but nil
from /Users/hidekazu/src/riak_test/vendor/bundle/ruby/2.1.0/gems/beefcake-1.0.0.pre2/lib/beefcake.rb:147:in `block in validate!'
```

I think as a bug since it is written to be able to README.

> Frequently, you might want a map with a Riak-assigned name instead of one you
> come up with yourself:
> 
> ```ruby
> map = Riak::Crdt::Map.new bucket, nil
> 
> map.registers['coat_pattern'] = 'tabby'
> 
> map.key #=> "2do4NvcurWhXYNQg8HoIR9zedJV"
> ```