Fix array to string cast error

- accidental trailing comma caused this (apparently that's a thing in
  Ruby...)
- cleaned up attribute assignments a little bit
  (persisted/validated/neither assignments)

(leoselig/codeocean#1)

app/controllers/exercises_controller.rb

@@ -70,8 +70,9 @@ class ExercisesController < ApplicationController
     begin
       user = user_for_oauth2_request()
       exercise = Exercise.new
-      exercise.from_proforma_xml(request.body.read)
+      request_body = request.body.read
-      exercise.update(:user => user)
+      exercise.from_proforma_xml(request_body)
+      exercise.user = user
       saved = exercise.save
       if saved
         render :text => 'SUCCESS', :status => 200

app/models/exercise.rb

@@ -109,8 +109,11 @@ class Exercise < ActiveRecord::Base
   def from_proforma_xml(xml_string)
     # how to extract the proforma functionality into a different module in rails?
     xml = Nokogiri::XML(xml_string)
-    self.title = xml.xpath('/root/p:task/p:meta-data/p:title/text()')[0].content
+    xml.collect_namespaces
-    self.description = xml.xpath('/root/p:task/p:description/text()')[0].content,
+    self.attributes = {
+      title: xml.xpath('/root/p:task/p:meta-data/p:title/text()')[0].content,
+      description: xml.xpath('/root/p:task/p:description/text()')[0].content
+    }
     self.execution_environment_id = 1
   end