Allowing arrays to be nested in numeric-key hashes

When params was of the form

    {
      :book => {
        :authors_attributes => {
          :'0' => ['William Shakespeare', '52'],
          :'1' => ['Unattributed Assistant']
        }
      }
    }

And its corresponding filter was

    {
      :book => {
        :authors_attributes => {
          :'0' => [],
          :'1' => []
        }
      }
    }

It resulted in params looking like this

    {
      :book => {
        :authors_attributes => {
          :'0' => nil,
          :'1' => nil
        }
      }
    }

This was due to the way hashes with all numeric keys are treated.
I just added a special case when a corresponding filter exists and is an empty
array.
Merge pull request #121 from simonc/121-arrays-within-a-hash

Permitting arrays within a hash