fix: treat a string passed to stringify as opaque content

[spokodev]

Problem

matter.stringify(file, data) documents the string form of file as "The content string to append to stringified front-matter" (and "appending that to the given string"). But when file is a string it runs matter(file), which parses the string as a document. If the body itself begins with the front-matter delimiter, that leading block is pulled out of the body and merged into the front matter — corrupting both content and data:

const matter = require('gray-matter');

const body = '---\nnot: real\n---\nHello world'; // intended as opaque content
const out  = matter.stringify(body, { title: 'Home' });
const back = matter(out);

back.data;    // { not: 'real', title: 'Home' }   <- body key leaked into front matter
back.content; // 'Hello world\n'                  <- '---\nnot: real\n---' lost from the body

A plain string (no leading delimiter) round-trips fine, so this is the documented happy path silently losing data, not misuse.

Fix

Wrap the string as { content: file, data: {} } instead of re-parsing it, so the body is appended verbatim under the new front matter. The empty data keeps stringify's existing no-data behaviour (a plain string with no data is returned unchanged). Object inputs and plain strings are unaffected.

Test

Added an assertion that a content string starting with a delimiter is appended (not parsed) and round-trips with clean data. It fails before the change and passes after; the full suite is green (76 passing).