v3.31.0 introduces bug

Nobody has claimed this yet.

The following code works fine in v3.30.0:

```
const query = {
where: {
or: [
{ username: username },
{ email: username }
]
}
};
self.userModel.findOne(query, function (err, user) {
```

As of v.3.31.0, I get the following error:
```
Error: The where clause "abc@xyz.com" is not an object
at Function.ModelUtils._coerce (C:\apps\flexit\node_modules\loopback-datasource-juggler\lib\model-utils.js:369:11)
at Function.ModelUtils._coerce (C:\apps\flexit\node_modules\loopback-datasource-juggler\lib\model-utils.js:552:20)
at Function.ModelUtils._coerce (C:\apps\flexit\node_modules\loopback-datasource-juggler\lib\model-utils.js:394:14)
at Function.ModelUtils._normalize (C:\apps\flexit\node_modules\loopback-datasource-juggler\lib\model-utils.js:177:8)
at Function.find (C:\apps\flexit\node_modules\loopback-datasource-juggler\lib\dao.js:1529:10)
at Function.findOne (C:\apps\flexit\node_modules\loopback-datasource-juggler\lib\dao.js:1782:8)
at Strategy._verify (C:\apps\flexit\server\boot\authentication.js:288:36)
at Strategy.authenticate (C:\apps\flexit\node_modules\passport-local\lib\strategy.js:90:12)
at attempt (C:\apps\flexit\node_modules\passport\lib\middleware\authenticate.js:361:16)
at authenticate (C:\apps\flexit\node_modules\passport\lib\middleware\authenticate.js:362:7)
```

Looks like something broke in the _coerce function.

Start in lib/model-utils.js at the _coerce calls around lines 369, 394, and 552, then follow normalization from lib/model-utils.js into findOne in lib/dao.js. Reproduce the provided or query on v3.31.0 and compare it with v3.30.0. Done means the query accepts the same object form without reporting the email value as a non-object.

Written by the indexing model from the issue text.