Options Class Name

The class name set in options[:class_name] is currently changed to singular (by .classify).

Example with the problem:
```ruby
has_one :profile_data, class_name: 'profile_data' 
```
gives the following error:
```ruby
NameError: uninitialized constant ProfileDatum
```
I expect it to use ProfileData if that is what I specify in options[:class_name]. In this commit, if the options[:class_name] is set then that class is used, otherwise it defaults to name.to_s.classify.

So:
```ruby
has_one :profile_data, class_name: 'profile_data' #will correctly use ProfileData
```