MINOR: cache: Do not store responses with an unknown encoding

If a server varies on the accept-encoding header and it sends a response
with an encoding we do not know (see parse_encoding_value function), we
will not store it. This will prevent unexpected errors caused by
cache collisions that could happen in accept_encoding_hash_cmp.
diff --git a/reg-tests/cache/vary_accept_encoding.vtc b/reg-tests/cache/vary_accept_encoding.vtc
index 59e819a..185ad05 100644
--- a/reg-tests/cache/vary_accept_encoding.vtc
+++ b/reg-tests/cache/vary_accept_encoding.vtc
@@ -292,6 +292,7 @@
 
        #
        # Unknown content-encoding
+       # The response should not be cached since it has an unknown content encoding
        #
        txreq -url "/unknown-content-encoding" -hdr "Accept-Encoding: gzip;q=0.8, deflate, first_encoding"
        rxresp
@@ -305,14 +306,6 @@
        expect resp.status == 200
        expect resp.http.content-encoding == "unknown_encoding"
        expect resp.bodylen == 119
-       expect resp.http.X-Cache-Hit == 1
-
-       # Different set of accepted encodings
-       txreq -url "/unknown-content-encoding" -hdr "Accept-Encoding: deflate, first_encoding"
-       rxresp
-       expect resp.status == 200
-       expect resp.http.content-encoding == "unknown_encoding"
-       expect resp.bodylen == 119
        expect resp.http.X-Cache-Hit == 0
 
        #