adding new validations and new tests to languages model

Signed-off-by: man13 <man13@inf.ufpr.br>

app/models/language.rb

 class Language < ActiveRecord::Base
-  has_many :learning_object
+  has_many :learning_objects
 
   validates_uniqueness_of :code
+  validates_presence_of :name, :code
 end

test/models/language_test.rb

+require 'test_helper'
+
+class LanguageTest < ActiveSupport::TestCase
+  should have_many :learning_objects
+
+  should validate_presence_of(:name)
+  should validate_presence_of(:code)
+
+  should validate_uniqueness_of(:code)
+end