"Missing id_token" when you navigate backwards after opening a Chirun link embedded in Canvas

[christianp]

To reproduce:

• click on a Chirun LTI link in Canvas
• wait for the page to load
• press the browser back button.

I think it's repeating the launch POST request, but with different parameters. The POST parameters are:

key	value
utf8	'✓'
authenticity_token	(a long string)
error	'launch_no_longer_valid'
error_description	'The launch has either expired or already been consumed'
state	(a long string)