I observed the following problem: `Transfer-Encoding` and
`Content-Length` headers should be mutually exclusive and because I use
chunked transfer, the `Transfer-Encoding` header is set in the response
while the `Content-Length` header is not. In case of a 304 during a
revalidation, the header contains Content-Length=0. Probably a proxy is
responsible for this, just like the comment "Some well-known proxies
respond with Content-Length=0, when returning 304" in the method
CachedHttpResponseGenerator::addMissingContentLengthHeader is saying. In
CacheEntryUpdater::mergeHeaders the Content-Length=0 is merged into the
cached entry, but the cached entry contains also a `Transfer-Encoding`
header, so in the cached entry these headers aren't mutually exclusive
anymore. Because of the `Transfer-Encoding` header the method
CachedHttpResponseGenerator::addMissingContentLengthHeader isn't fixing
the `Content-Length` header and Content-Length=0 causes returning null
instead of the cached content. IMHO the `Content-Length` header should
not be merged into the cached response in case of a 304, at least if the
cached entry contains a `Transfer-Encoding` header.
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