Precaching doesn't work properly in Safari when offline

**Library Affected**:
workbox-precaching

**Browser & Platform**:
Safari (both OSX and iOS)

**Issue or Feature Request Description**:
Here is a minimal example of a tiny page which works fine in Chrome, but doesn't work in Safari.

The only workaround I found is to access precaching via `matchPrecache` in catch handler.

Please, take a look at this repo, I described step-by-step reproduction steps there.

https://github.com/mdevils/workbox-offline-safari