Use response.status === 200 instead of response.ok for cacheability

[jeffposnick]

Library Affected:
workbox-core, workbox-strategies

There are a couple of places where we default to checking response.ok to determine whether a given response is cacheable or not. (This can be overridden by workbox-cacheable-response, but we're talking about the defaults.)

response.ok is true iff 200 <= response.status <= 299. Unfortunately, there are some scenarios where we don't want to cache responses in that range, like when response.status === 206 (for partial content). Attempting to cache that type of response will fail as per the Cache Storage API.

Rather than go with response.ok && response.status !== 206 as the default criteria, I think we should just switch to response.status === 200 as the default, unless folks have a strong use case for caching other 2xx status responses by default. We'll continue to allow opting-in to custom cacheability criteria using workbox-cacheable-response.