RemoteStore::addToStore(): Pass content-addressability assertion

... and use this in Downloader::downloadCached(). This fixes

  $ nix-build https://nixos.org/channels/nixos-16.09-small/nixexprs.tar.xz -A hello
  error: cannot import path ‘/nix/store/csfbp1s60dkgmk9f8g0zk0mwb7hzgabd-nixexprs.tar.xz’ because it lacks a valid signature

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index 25ccd7d0b5..a56fd6922a 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -612,6 +612,7 @@ Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpa
                 Hash hash = hashString(expectedHash ? expectedHash.type : htSHA256, *res.data);
                 info.path = store->makeFixedOutputPath(false, hash, name);
                 info.narHash = hashString(htSHA256, *sink.s);
+                info.ca = makeFixedOutputCA(false, hash);
                 store->addToStore(info, sink.s, false, true);
                 storePath = info.path;
             }