Ensure that robots.txt has a text/plain Content-Type
Ensure that robots.txt has a text/plain Content-Type

Twitter won't even consider it without [1]. Currently it's a
`text/html`.

[1] https://developer.twitter.com/en/docs/tweets/optimize-with-cards/guides/getting-started


Former-commit-id: 28ac85ff5fd6caf57da974aff2a1af97f8943ef3
brandur/sorg,brandur/sorg,brandur/sorg,brandur/sorg,brandur/sorg