Support top level arrays

[Davenchy]

I think you can support top level arrays now as specified in BSON spec.

• Array: The document for an array is a normal BSON document with integer values for the keys, starting with 0 and continuing sequentially.

For example, the array ['red', 'blue'] would be encoded as the document {'0': 'red', '1': 'blue'}. The keys must be in ascending numerical order.