Preserve arrays under unflatten when safe and object are both set

[spokodev]

What

The README says safe makes "both flatten and unflatten preserve arrays and their contents". But when object: true is also set, unflatten turns a preserved array back into a plain object:

const opts = { safe: true, object: true }
const flat = flatten({ tags: ['red', 'green', 'blue'] }, opts) // { tags: ['red','green','blue'] } (array kept)
unflatten(flat, opts)
// actual:   { tags: { '0': 'red', '1': 'green', '2': 'blue' } }   <- no longer an array
// expected: { tags: ['red', 'green', 'blue'] }

safe and object are orthogonal (safe = keep existing arrays; object = don't turn numeric-keyed objects into arrays) and combining them is legitimate, but object overrides safe's array preservation. safe alone round-trips correctly.

Root cause

unflatten never truly preserves arrays — it only rebuilds them as a side effect of the numeric-key → array branch (typeof key2 === 'number' && !opts.object). Its "messy objects" reduce re-flattens every container value, including a safe-kept array, back into the flat keyspace; with object: true the rebuild produces a plain object instead of an array. safe is never consulted in unflatten to keep an already-array value intact.

Fix

In the reduce, pass a safe array through as a leaf instead of re-flattening it, so it survives the reconstruction unchanged regardless of object.

Tests

Added a .safe case asserting { safe: true, object: true } round-trips { tags: ['red','green','blue'] } as an array. It fails on master and passes with the fix; the suite and standard are green.