(fix for edmodo server errors) Appends text/html as an acceptable response type if not present in the HTTP_ACCEPT header. This should stop malformed user agents from generating 'Missing template' errors on pages.
make http header fix specific to Edmodo user agent
add test for edmodo header fix
Merge branch 'master' of https://github.com/code-dot-org/dashboard into edmodo_fix_4

Conflicts:
	app/controllers/application_controller.rb
Merge pull request #334 from code-dot-org/edmodo_fix

fix for edmodo server errors