incoming json support

[nvdk]

We had a problem with incoming json on access token request. This is fixed in here: https://github.com/nvdk/rack-oauth2-server/commit/6358b7da9903ff6c8db661ade65762f1ccf902b5

in your tree, this would be

       if options.access_token_path == request.path
            if env['CONTENT_TYPE'] =~ /^application\/json/ && request.post?
               env.update('rack.request.form_hash' => ActiveSupport::JSON.decode(env['rack.input'].read),'rack.request.form_input' => env['rack.input'])
            end
end