Only pass `:distinct` if it was specified.

This prevents `:distinct` deprecation warnings when the option was not passed at all.
The prevented deprecation looks like:

```
DEPRECATION WARNING: The :distinct option for `Relation#count` is deprecated. Please use `Relation#distinct` instead. (eg. `relation.distinct.count`). (called from calculate at /Users/max/Projects/activerecord-deprecated_finders/lib/active_record/deprecated_finders/relation.rb:84)
```

This issue was discovered while working on: https://github.com/rails/rails/issues/13165
Merge pull request #20 from senny/dont_always_warn_for_distinct_option

Only pass `:distinct` if it was specified.