Chrome 60: Error parsing 'integrity' attribute ('undefined').

[piotr-cz]

Library Affected:
7.1.0

Browser & Platform:
Chromium v60.0.3112.0

Issue or Feature Request Description:

Errors in console:

(11) Error parsing 'integrity' attribute ('undefined'). The hash algorithm must be one of 'sha256', 'sha384', 'sha512', followed by a '-' character.

It seems that Old Chromium versions show errors when fetching Request with integrity: undefined.
One of such example:

workbox/packages/workbox-precaching/src/PrecacheController.ts

Lines 212 to 216 in 9e69c42

	const request = new Request(url, {
	integrity,
	cache: cacheMode,
	credentials: 'same-origin',
	});

Fix is to omit the integrity attribute when it's undefined.

Errors are not present in Chromium 64+