Parse arrays with a single item and no numbering - #2

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",
        }],
    }