Fixing failing rspec because of oauth2_provider engine

Because of the inclusion of oauth2_provider engine, all navigation routes had to be prepended with either 'main_app' or 'oauth_engine'. This caused few specs to fail because of the way urls were stubbed before. Changed it by including a EngineUrlHelper which stubs all header links and oauth2_provider links.
Fixing ApplicationHelper spec with proper routes stubbing on main_app
Fixing application_nav spec with correct ids because PIPELINES now use rails routes instead of the previous hardcoding of URL
Merge pull request #431 from sachinsudheendra/rspec-fix-after-oauth2-provider

Rspec fix after oauth2 provider merge into master