Parse arrays with a single item and no numbering

[zvyn]

This patch fixes an issue when integrating with
mozilla-services/react-jsonschema-form where only one item is in an
array.

Example:

{
    "nested[][nested_key_1]": "nested_value_1",
    "nested[][nested_key_2]": "nested_value_2",
}

With this patch the result is (instead of the original example above):

{
    "nested": [{
    "nested_key_1": "nested_value_1",
    "nested_key_2": "nested_value_2",
    }],
}

[sheppard]

This seems like a useful change, thanks!