Added devise.omniauth initializer explicit position requirements.

It is obvious that this initializer should be executed before Rails build_middleware_stack as Omniauth is build on middleware.
Also it is obvious that we need that initializer to be executed after all config/initializers/* files (that is where devise.rb usually is).

test/rails_test.rb

@@ -0,0 +1,9 @@
+require 'test_helper'
+
+class DeviseTest < ActiveSupport::TestCase
+  test 'correct initializer position' do
+    initializer = Devise::Engine.initializers.detect {|i| i.name == 'devise.omniauth' }
+    assert_equal  :load_config_initializers, initializer.after
+    assert_equal  :build_middleware_stack, initializer.before
+  end
+end