Fixes #717.

Problem

When Writable is used without the optional Keys argument, it doesn't only strip readonly — it also changes the object's structure. The clearest symptom is an index signature: Writable<{readonly [key: string]: number}> currently resolves to {[x: string]: number; [x: number]: number} — a spurious numeric index signature gets added.

This happens because the default (all-keys) case still goes through the Except<BaseType, Keys> & {-readonly [K in keyof Pick<BaseType, Keys>]: …} rebuild, and that Except/Pick round-trip does not faithfully reproduce the original shape.

Fix

As suggested in the issue thread ("add an overload when that parameter is not passed and use the simple handling"), the default case now uses a plain homomorphic mapped type:

{-readonly [KeyType in keyof BaseType]: BaseType[KeyType]}

This only removes the readonly modifier and otherwise preserves the input structure exactly, including index signatures — matching the simpler community implementations referenced in the issue. The explicit subset-of-Keys path (Writable<T, 'a' | 'b'>) is left completely unchanged, so all existing behavior and tests are preserved.

The all-keys case is detected with IsEqual<Keys, keyof BaseType> (already used internally by Except), which is true both when Keys is omitted and when the full key set is passed explicitly — semantically identical intents.

Tests

Added two tsd assertions to test-d/writable.ts:

• Writable<{readonly [key: string]: number}> → {[key: string]: number}
• Writable<{readonly [key: string]: number; readonly foo: number}> → {[key: string]: number; foo: number}

Both fail on the current code (the result carries the extra [x: number]: number index signature) and pass with the fix. Verified with git stash on the source change. The full npm test (tsc + tsd + xo + linter) passes.