flatten with depth=1 is not properly applied to the first element

[bd82]

Expected result would be [1, 2, 3]
But the actual is [[1], 2, 3]

Code snippet:

const flatten = require('just-flatten-it')

flatten([[1], [2], 3], 1);

[superhit0]

@angus-c I opened a PR for the fix, please let me know your thoughts on this - #577