fix: reset the buffer because old data is still there (google#89)

feat: add sxg_fallback_host option (google#87)

* feat: add sxg_fallback_host

Update non-LTS release to the latest. (google#86)

Update 3 github workflows to use 2 latest LTS releases and 1 latest
non-LTS release of Ubuntu. Updated clang-format to use ubuntu-latest
instead of 18.04.

use requested path instead of resolved path for embedding URL (google#78

)

* use requested path instead of resolved path for embedding URL

use time_t for lifespan

add nosniff outer header automatically (google#62)

* cleanup outer headers

* update e2e test

* use = instead of == in test command

* clear trailers

add mutex to save context (google#46)

* add mutex to save context

defeat minor compiler warning (google#44)

Don't embed subresource preload header when nginx failed to fetch (go…

…ogle#37)

* do not embed preload header to subresource when nginx failed to fetch the subresource

* unrecoverable error should use NGX_LOG_EMERG

* do not embed integrity of subresource which HTTP status is not "200 OK"

error message should be appeared with NGX_LOG_EMERG (google#33)

* unrecoverable error should use NGX_LOG_EMERG