find_by_fixnum expects string parameter

The generated find_by view doesn't know that my field is a Fixnum. Is that possible with simply_stored?

```
class Product 
  include SimplyStored::Couch
  property :quantity, :type => Fixnum, :default => 0
  validates_numericality_of :quantity, :allow_nil => true
end

Product.find_by_quantity(123) # => nil
Product.find_by_quantity("123") # => #<Product (etc.) quantity: 123 >
```
