Fix undefined method issue

For some reason this code ```parsed = ::JSON.parse(body).with_indifferent_access``` at the line below doesn't work in my environment as the _with_indifferent_access_ is undefined for Hash (see attachment). This pull request requires 'active_support/core/ext/hash' and it solves the problem for me.

https://github.com/vkvelho/endpoint_base/blob/52a8d2af7f113844fb71eb2e8bcfeebaaf452b38/lib/endpoint_base/concerns/param_processor.rb#L24

This was very hard to debug, because all I got was 406 raised by "rescue Exception". If I used Honeybadger or Airbrake I got the actual error, but without those services... nothing but 406.

![with_indifferent_access_error](https://cloud.githubusercontent.com/assets/1226045/5414244/18a921fc-8226-11e4-8ef6-d53a1474669a.png)