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, 6 insertions, 0 deletions

diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index b5934a0d12..401b001b2d 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -676,6 +676,12 @@ Strings ValidPathInfo::shortRefs() const
 }
 
 
+std::string makeFixedOutputCA(bool recursive, const Hash & hash)
+{
+    return "fixed:" + (recursive ? (std::string) "r:" : "") + hash.to_string();
+}
+
+
 }