Update adapter to work with author page queries

[jacklenox]

The WordPress.com Elasticsearch Post Document Schema doesn't have post_author.user_nicename, so this PR maps it to author_login.

This should work in all situations on WordPress.com because we prevent email addresses etc. being used as usernames during signup.

Fixes #46

[kasparsd]

Thanks @jacklenox. Maybe we could map user_login too?

'post_author.user_login' => 'author_login',

Update: looks like user_login isn't used anywhere by the ES filter builder so we don't really need it.

[jacklenox]

Oh, thanks @kasparsd, yes good idea! I hadn't realised you had updated your PR. Apologies, I didn't mean to try to supersede it! I'll update this now with that too.

[jacklenox]

Oh haha, I really should stop firing from the hip without reading things fully!

Update: looks like user_login isn't used anywhere by the ES filter builder so we don't really need it.

☹️

[mboynes]

Thanks for putting this together!