Secure- to Sec- in HTTP header fields

[Jxck]

Seems typo.

[pfreitag]

See #59

[drubery]

Yep, not a typo. Given that Sec- doesn't actually do anything for response headers, multiple parties felt it was inappropriate to include. So we're using Secure-Session- as the common prefix for DBSC headers. Only Sec-Secure-Session-Id gets the Sec- prefix right now, since I felt there was some value in the Fetch forbidden header list on that one