fix: preserve non-enumerable properties in cached results

[mixelburg]

Summary

Fixes two caching bugs:

1. .matter and .language lost on cached calls (#174)

The matter and language properties are defined as non-enumerable via Reflect.defineProperty in toFile.js. When the cache returns a shallow copy via Object.assign({}, cached), non-enumerable properties are skipped. This means on the second (cached) call, file.matter and file.language are undefined.

const matter = require('gray-matter');
matter('---\ntitle: Title\n---').matter // '\ntitle: Title'
matter('---\ntitle: Title\n---').matter // undefined (BUG)

Fix: Explicitly copy matter and language after Object.assign, following the same pattern already used for orig.

2. Invalid front-matter was cached (#166)

The file object was cached before parseMatter ran, so if parsing threw an error (e.g. invalid YAML), the raw input stayed in the cache. The second call returned a wrong result instead of throwing again.

Fix: Moved matter.cache[file.content] = file to after parseMatter succeeds.

Fixes #174, fixes #166