Cryptic error for chttpd_auth_cache:username

I apologize if this is documented elsewhere, but I am seeing a lot of the error below on our production server. Is this okay? If not, is this an error in our code or in couchdb? To my knowledge none of our views/validation use a function called "username". Thank you in advance for your time.

```
[notice] 2018-12-19T23:46:43.281783Z couchdb@127.0.0.1 <0.332.0> -------- chttpd_auth_cache changes listener died no function clause matching chttpd_auth_cache:username(<<"e43caea614fb6e2250df47e4da043d64">>)(line:18$
[error] 2018-12-19T23:46:43.281838Z couchdb@127.0.0.1 emulator -------- Error in process <0.8490.6970> on node 'couchdb@127.0.0.1' with exit value:
{function_clause,[{chttpd_auth_cache,username,[<<"e43caea614fb6e2250df47e4da043d64">>],[{file,"src/chttpd_auth_cache.erl"},{line,183}]},{chttpd_auth_cache,changes_callback,2,[{file,"src/chttpd_auth_cache.erl"},{line,$
```

## Expected Behavior

No notice or error is logged

## Your Environment
This is running on Couch DB 2.2.0, Node 10.9.0, Ubuntu 16.04.5

Start by reading src/chttpd_auth_cache.erl around line 183 and trace how the changes listener calls username/2 with the logged value. Use the CouchDB 2.2.0 environment details and the supplied log to understand the failure, then verify that the changes listener no longer emits the notice or error.

Written by the indexing model from the issue text.