Client password printed in logs in plaintext

I'm finding that under an amount of load this plugin is beginning to have requests to the configured HTTP endpoint timeout. This is probably either some resource issue, or some issue with my API implementation.

So that's probably my problem, but the reason I'm raising this issue is that when this occurs, the entire request URL is printed in the EMQX logs - including the client's password.

Here is an example log output:

```
{"log":"(emqx@emqx-1.emqx-headless.stream.svc.cluster.local)1> 2021-05-17 03:04:20.825 [error] <<"57d06312-67ef-4c95-8d63-0e606da765c2">>@10.49.10.178:56094 [Hooks] Failed to execute {emqx_auth_http,check,[#{auth_req => {http_request,post,"/auth",[{<<"content-type">>,<<"application/x-www-form-urlencoded">>}],[{"username","%u"},{"password","%P"},{"clientid","%c"},{"ipaddress","%a"}],5000},pool_name => emqx_auth_http,super_req => undefined}]}: {{timeout,{gen_server,call,[<0.1723.0>,{post,{"/auth",[{<<"content-type">>,<<"application/x-www-form-urlencoded">>}],<<"username=REDACTED&password=REDACTED&clientid=REDACTED&ipaddress=10.49.10.178">>},5000},6000]}},[{gen_server,call,3,[{file,"gen_server.erl"},{line,223}]},{emqx_auth_http_cli,do_request,5,[{file,"emqx_auth_http_cli.erl"},{line,50}]},{emqx_auth_http,check,3,[{file,"emqx_auth_http.erl"},{line,45}]},{emqx_hooks,safe_execute,2,[{file,"emqx_hooks.erl"},{line,164}]},{emqx_hooks,do_run_fold,3,[{file,"emqx_hooks.erl"},{line,143}]},{emqx_access_control,authenticate,1,[{file,"emqx_access_control.erl"},{line,77}]},{emqx_channel,auth_connect,2,[{file,"emqx_channel.erl"},{line,1181}]},{emqx_misc,pipeline,3,[{file,"emqx_misc.erl"},{line,107}]}]}
```

Start with emqx_auth_http_cli:do_request/5 in emqx_auth_http_cli.erl, then follow the timeout stack shown through emqx_auth_http:check/3. Reproduce or inspect the timeout logging path and confirm that completed error logs contain no client password or other request credentials.

Written by the indexing model from the issue text.