MultiJson::Adapters::Oj#load ignores :symbolize_keys when it's false

`MultiJson::Adapters::Oj#load` ignores the `:symbolize_keys` option when it's false. It matters if I set the default Oj behaviour to symbolize keys: 

``` ruby
Oj.default_options = { symbol_keys: true }
```
and then try to parse one string without symbolizing:

``` ruby
MultiJson.load('{"a":1}', symbolize_keys: false)
# => {:a=>1}
```

This pull request fixes the problem.