Fix ActiveResource undefined method `first' for nil if resource unavailable in belongs_to_remote

Related to https://github.com/denyago/remote_association/issues/6

Given: `Postcode` is `ActiveResource` model
When resource is unavailable next code raises undefined method `first' for nil:NilClass
 
```ruby
Postcode.find(:first, params: {"search[code_in]"=>["AB1"]})
```

This code works fine:
```ruby
Postcode.find(:all, params: {"search[code_in]"=>["AB1"]}).try(:first)
```