libutil: allow decompression with none/empty method

The S3 store relies on the ability to be able to decompress things with
an empty method, because it just passes the value of the Content-Encoding
directly to decompress.

If the file is not compressed, then this will cause the compression
routine to get confused.

This caused NixOS/nixpkgs#120120.

src/libutil/compression.cc

@@ -186,7 +186,9 @@ struct BrotliDecompressionSink : ChunkedCompressionSink
 
 ref<std::string> decompress(const std::string & method, const std::string & in)
 {
-    if (method == "br") {
+    if (method == "none" || method == "")
+        return make_ref<std::string>(in);
+    else if (method == "br") {
         StringSink ssink;
         auto sink = makeDecompressionSink(method, ssink);
         (*sink)(in);