Fix #as_json glitch caused by JSON and Rails

[changa]

This should solve #340.

When requiring Rails, as_json somehow ends up with a
JSON::Ext::Generator::State instance as arguments, instead of a plain old Hash,
which causes this:

2.0.0 (main):0 > require "active_model_serializers"
=> true
2.0.0 (main):0 > require "json"
=> true
2.0.0 (main):0 > JSON.dump  ActiveModel::Serializer.new(Object.new)
=> "\"#<ActiveModel::Serializer:0x007f7a2101a458>\""
2.0.0 (main):0 > require "rails"
=> true
2.0.0 (main):0 > JSON.dump  ActiveModel::Serializer.new(Object.new)
NoMethodError: undefined method `fetch' for #<JSON::Ext::Generator::State:0x007f7a2093bfb0>
from /home/drr/code/af/map/code/.bundle/ruby/2.0.0/gems/active_model_serializers-0.8.1/lib/active_model/serializer.rb:341:in `as_json'`'`

[coveralls]

Coverage remained the same when pulling 55425c5 on changa:fix_json_glitch_with_rails into 919bb38 on rails-api:master.

[spastorino]

This works fine in current master. Thanks for helping.