When used in combination with GzipResourceResolver, the
CachingResourceResolver does not properly cache results, since it only
takes the request path as a input for cache key generation.
Here's an example of that behavior:
1. an HTTP client requests a resource with `Accept-Encoding: gzip`, so
the GzipResourceResolver can resolve a gzipped resource.
2. the configured CachingResourceResolver caches that resource.
3. another HTTP client requests the same resource, but it does not
support gzip encoding; the previously cached gzipped resource is still
returned.
This commit uses the presence/absence of gzip encoding support as an
input in cache keys generation.
Issue: SPR-12982
bb3f26483b