AutoScaling Group initializer attribute assignment

Attempts to fix issue #1284.  I ran into the problem when passing min_size and max_size to the constructor of an AWS AutoScaling Group. It would not set the attributes because the "XML-style" name was being set first and overriding the subsequent "ruby-style" name.  More specifically, this was happening:

```ruby
group = Fog::AWS::AutoScaling::Group.new({min_size: 1, max_size: 2})
group.min_size # => 0
group.max_size # => 0
```

Following your advice from #1284, I set the default values using attribute setters instead of attributes['AttrName'] ||=.  Also, since this happens before calling super, I just used direct assignment instead of the ||= conditional assignment.

I'm not too familiar with shindo, and I'm also not sure if the issue has more subtle implications than what I've tested here, so let me know if this needs some work.