Global options have priority over adapter options and defaults

I thought that if we decide to bring back `quirks_mode` as a default in json_{gem,pure} adapters, the first that user will try to do to cancel it would be something like this:

```ruby
MultiJson.load_options = MultiJson.dump_options = { quirks_mode: false }
```

And that will not work as expected because adapter-specific options (with quirks_mode enabled by default) have bigger priority.

That pull-request changes that and put global options over adapter-specific and defaults.