fix keyword error for required_realm

The keyword on request validator is `required_realm`, but it is called with a keyword `valid_realms` which could raise a KeywordError.

**Suggestion**:

There are `realm` and `realms`, it is not clear to see if it is a list or not. Let's make `realm` a string with whitespace and `realms` a list. Which means:

```
realm = 'email article'
realms = ['email', 'article']
```