Subscription Widget Email Confirmation link errors when hosting with Pantheon

[njandreau]

I have discovered that if you are hosting on Pantheon the email confirmation link (opt-in) that is sent to a newsletter subscriber errors out. A user is still subscribed but they never get the confirmation page or any of the virtual pages due to a redirect loop. It's caused by Pantheon replacing __sg_api=1 with __sg_api=PANTHEON_REPLACED. This is some sort of caching function on Pantheon. They make exceptions for Google UTM codes but not Sendgrid. I fixed mine by removing one underscore for every instance of __sg_api in class-sendgrid-mc-optin.php.