fix keyword error for required_realm

[lepture]

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']

[lepture]

Merging #189 instead.

I would keep it open, if you dislike 189, we can talk it here.