Search can be case sensitive in some scenarios

Illustrates a bug found when developing #1557 

```sh
Failures:
  1) PublicBody  when indexing public bodies with Xapian is not case sensitive
     Failure/Error: xapian_object.results[0][:model].should == public_bodies(:humpadink_public_body)
     NoMethodError:
       undefined method `[]' for nil:NilClass
     # ./spec/models/xapian_spec.rb:54:in `block (2 levels) in <top (required)>'
```

Doesn't find "Department for Humpadinking" when searching for "Humpadink", but is successful when searching for "humpadink". Likely something to do with the stemming strategy.