write json-like data for Jubatus::Common::Datum.new

permit symbol hash keys in Jubatus::Common::Datum.new

```ruby
old_data = Jubatus::Common::Datum.new({ 'name' => 'katsyoshi', 'age' => 30})
new_data = Jubatus::Common::Datum.new({name: 'katsyoshi', age: 30})
```