Always serialize PHP indexed arrays as JSON arrays, not objects

When an indexed array has a removed item at the
middle (i.e. not sequential indexes) it is not anymore 
serialized as JSON array.
For example in Symfony2, when using Collection Form Type and removing
an item (with an ArrayCollection it's the same problem).

The array inner type was also not used during serialization with form "array<T>", but only with "array<K, V>".