Accept JSON for incoming access token request

cf. https://github.com/nvdk/rack-oauth2-server/commit/6358b7da9903ff6c8db661ade65762f1ccf902b5
Ruby-2: prevent fail on require iconv

Iconv has been removed from Ruby 2.0.
I dont see any Iconv call on the code maybe we can remove it ?
Test: fix task all to use framework

Before the framework was always Rails due to task :default
Merge remote-tracking branch 'spk/ruby-2'
Merge remote-tracking branch 'donedotcom/json_access_token'