Don't require auth on login attempts

[kocolosk]

Overview

Previously with require_valid_user=true configured a user would need to supply Basic auth credentials in order to login via the _session endpoint (or have some otgher Catch-22 way of using an existing session). This patch makes it so that any attempt to POST to _session is allowed to proceed.

Testing recommendations

Try to POST to _session with require_valid_user = true and don't supply any additional basic auth header. Without this patch you would get rejected.

Related Issues or Pull Requests

Closes #1947.

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation

[nickva]

 curl -v http://127.0.0.1:5984/_session -H 'Content-Type:application/x-www-form-urlencoded' -d 'name=adm&password=pass'
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 5984 (#0)
> POST /_session HTTP/1.1
> Host: 127.0.0.1:5984
> User-Agent: curl/7.54.0
> Accept: */*
> Content-Type:application/x-www-form-urlencoded
> Content-Length: 22
>
* upload completely sent off: 22 out of 22 bytes
< HTTP/1.1 200 OK
< Cache-Control: must-revalidate
< Content-Length: 44
< Content-Type: application/json
< Date: Thu, 21 Nov 2019 19:16:20 GMT
< Server: CouchDB/3.0.0-8904f6f0f (Erlang OTP/21)
< Set-Cookie: AuthSession=YWRtOjVERDZFMjg1OqrvTS-Z7L8h67Pe8lE3hvHH4y5R; Version=1; Expires=Thu, 21-Nov-2019 19:26:21 GMT; Max-Age=600; Path=/; HttpOnly
<
{"ok":true,"name":"adm","roles":["_admin"]}

curl -s  --cookie AuthSession=YWRtOjVERDZFMjg1OqrvTS-Z7L8h67Pe8lE3hvHH4y5R  -H "X-CouchDB-WWW-Authenticate: Cookie" http://127.0.0.1:5984/_node/_local/_config/chttpd | jq
{
  "max_db_number_for_dbs_info_req": "100",
  "port": "5984",
  "prefer_minimal": "Cache-Control, Content-Length, Content-Range, Content-Type, ETag, Server, Transfer-Encoding, Vary",
  "backlog": "512",
  "socket_options": "[{sndbuf, 262144}, {nodelay, true}]",
  "require_valid_user": "true",
  "server_options": "[{recbuf, undefined}]",
  "bind_address": "127.0.0.1"
}