Cache: only re-encode if maximum quality wasn't used originally

[OliverJAsh]

It seems that this module will asynchronously re-encode at the best/maximum quality, even if the maximum quality was used originally. IIUC, this means that if you ask for the maximum quality, shrink-ray will end up compressing the resource twice at the maximum quality.

I think we need to add a guard around the re-encode, so that it only runs if maximum quality wasn't already used:

shrink-ray/index.js

Lines 411 to 422 in f7cfcc7

	// now asynchronously re-encode the entry at best quality
	const result = new BufferWritable();
	new BufferReadable(buffer)
	.pipe(getBestQualityReencoder(coding, zopfli))
	.pipe(result)
	.on('finish', function () {
	const itemInCache = lru.peek(key);
	if (itemInCache) {
	itemInCache.buffer = result.toBuffer();
	}
	});

[OliverJAsh]

Hi @Alorel would you accept a PR to fix this?