Identical calls to `matter` should produce the same result

[Michael Overmeyer]

Identical, successive calls to matter return different results.

Example code:

const input = `---
foo: bar
---
Tada!`;

const parsed = matter(contents);
const parsed_again = matter(contents);
console.log(parsed.language) // => yaml
console.log(parsed_again.language) // => undefined

I understand from this comment that it's caching based on the input string, and I can cache bust by passing in an empty options hash as a workaround.

In this case, I'm fine with it caching (even though it was unexpected and IMO should at least be documented). However, the cache should cache the resulting object exactly, so that identical calls to matter always return the same result.