ssl_certificate(): auto-ssl: failed to run do_ssl

Not sure if this is an issue here or `lua-resty-http` issue but I'm getting this error in a clean install:

```
2020/05/16 19:08:01 [error] 17897#17897: [lua] ssl_certificate.lua:310: ssl_certificate(): auto-ssl: failed to run do_ssl: /usr/local/share/lua/5.1/resty/http.lua:949: bad argument #2 to 'set_keepalive' (number expected, got nil), context: ssl_certificate_by_lua*, client: , server: 0.0.0.0:443
```

These are the versions I'm using:
```
$ nginx -v
nginx version: openresty/1.9.15.1
```

```
$ sudo luarocks list

Installed rocks:
----------------

lua-resty-auto-ssl
0.13.1-1 (installed) - /usr/local/lib/luarocks/rocks

lua-resty-http
0.15-0 (installed) - /usr/local/lib/luarocks/rocks

shell-games
1.1.0-1 (installed) - /usr/local/lib/luarocks/rocks

```

I was able to find a temporary solution by patching `/usr/local/share/lua/5.1/resty/http.lua:949` and hardcode the values:

```lua
local ok, err = self:set_keepalive(6000, 30) --params.keepalive_timeout, params.keepalive_pool)
```

Start by comparing ssl_certificate.lua:310 with resty/http.lua:949 using the reported OpenResty 1.9.15.1, lua-resty-auto-ssl 0.13.1-1, and lua-resty-http 0.15-0 versions. Reproduce the clean-install failure and trace the keepalive parameters; done means auto-ssl completes do_ssl without the set_keepalive argument error.

Written by the indexing model from the issue text.