feat(cache): added ability to control which response headers are stored

[tompahoward]

What

Provides the cachedResponseHeaders configuration option, which can be used to adjust which HTTP response headers are stored in the cached and then populated in cached responses.

If the cachedResponseHeaders option is not provided, then a default set of headers are stored as per the existing behaviour, maintaining backwards compatibility.

To allow users to store headers in addition to the default set of headers, the default set of headers is available from mfh.CACHED_RESPONSE_HEADERS

Why

Some HTTP servers provide important response headers to clients that are not included in the set of headers that are stored in the cache, such as the HTTP Link header. Failure to include these headers in cached responses can break the clients that expect and use these headers.

References

Related to #62

[wraithgar]

This is a great direction, thank you.

Can we move these into lib/options.js so that the defaults method can set them?

[nlf]

closing in favor of #225