Fix mongoization of Money when using infinite_precision.

I ran into a problem using `Money.infinite_precision = true` in conjunction with Mongoid:

```ruby
[36] pry(main)> Money.new(BigDecimal.new('12345.6')).mongoize
=> {:cents=>#<BigDecimal:7fe55a5723e8,'0.123456E5',18(18)>, :currency_iso=>"USD"}
```
When saving the model using this kind of money, Moped (or Mongoid) threw an error like this:
```
undefined method `__bson_dump__' for #<BigDecimal:7fe55a5723e8,'0.123456E5',18(18)>
```

It was caused by not mongoizing the values (particularly the cents) in the hash returned by `Money#mongoize`.

This PR fixes it and adds tests for it!