PostCSS aims for byte-to-byte equal output, but the stringifier dropped a
leading byte order mark: `parse('a{}').toString()` returned `a{}`. The
BOM is already detected (`Input#hasBOM`) but was never re-emitted, so the
`bom.css` round-trip test had to be skipped.
Emit the BOM in `Stringifier#root` when the source had one, and re-enable the
previously skipped test.