Preserving adapter declaration for repository

This pull request addresses a very specific behavior as documented in
the [README](https://github.com/lotus/model/blob/master/README.md#adapter).

It also appears that there are several paths to setting a repository's
adapter. Below is an excerpt from `Mapper#load!`

```console
./lib/lotus/repository.rb:211:in `adapter='
./lib/lotus/model/mapping/collection.rb:364:in `configure_repository!'
./lib/lotus/model/mapping/collection.rb:352:in `load!'
./lib/lotus/model/mapper.rb:105:in `block in load!'
./lib/lotus/model/mapper.rb:103:in `each_value'
./lib/lotus/model/mapper.rb:103:in `load!'
```

Closes lotus/model#84