WFE: Forbid GET to Certificates in -strict. (#174)
WFE: Forbid GET to Certificates in -strict. (#174)

Also fix cases where multiple calls to `response.WriteHeader` occurred
because there was an extra explicit call to set a response status header
before calling `wfe.sendError` which sets the same header.
shred/pebble,letsencrypt/pebble,shred/pebble,letsencrypt/pebble,letsencrypt/pebble