Fix the case when using order_by_function with Solr strdist

When using the [Solr strdist](http://wiki.apache.org/solr/FunctionQuery#strdist) FunctionQuery with the new ```order_by_function``` it is required to include a distance measure to use, but sunspot consider each param as it were fields, this pull request fixes that situation.

```
valid_measures = [:jw, :edit, :ngram]
```