ignored string options

https://github.com/laserlemon/simple_oauth/blob/master/lib/simple_oauth/header.rb#L83 does:
```ruby
    def attributes
      ATTRIBUTE_KEYS.inject({}) { |a, e| options[e] ? a.merge(:"oauth_#{e}" => options[e]) : a }
    end
```
does reject all `options` keys that are not in `ATTRIBUTE_KEYS` ... there is no warning and it takes time to debug, throwing `Exception` would be much safer, maybe add a flag to allow ignoring but make it  fail by default?