Fix for multiple cacheId prefixes in the precache handler

[jeffposnick]

R: @addyosmani @gauntface
CC: @HenrikJoreteg @liuruoran88

This fixes #714 by explicitly changing the method call inside of WorkboxSW to indicate that for the internal CacheOnly handler use for precached requests, cacheId shouldn't be prepended when determining the corresponding cache name.

I decided to go with that approach rather than making a chance in the RequestWrapper constructor that checked to see if cacheId was already present in the cacheName, since you could theoretically imagine calling

new RequestWrapper({
  cacheName: 'my-prefix',
  cacheId: 'my-prefix',
}});

In that situation, the user would expect the effective cache name to be 'my-prefix-my-prefix', and we'd want to still allow that.

[gauntface]

I'm ok with the PR going in, but we really need to sanitise the cache naming stuff. Every time I try and reason with it in my head, it quickly falls down to - no idea, it's from the request wrapper, which comes from different places depending on what you are doing and that will depend on what is passed to it.

[jeffposnick]

The interplay of cacheId and cacheName, while useful, definitely complicates the logic, too.