filetransfer: fix decompression regression from 121edecf

121edecf65 added a new state field to
carry over content encoding settings from transfer to sink creation, but
never actually set that field.

Change-Id: I714b2efe745561e851b78a4791479b3501db8c72

src/libstore/filetransfer.cc

@@ -761,6 +761,10 @@ struct curlFileTransfer : public FileTransfer
                     state.wait_for(state->request, std::chrono::seconds(10));
                 }
 
+                if (state->encoding.empty()) {
+                    state->encoding = transfer.encoding;
+                }
+
                 /* Append data to the buffer and wake up the calling
                 thread. */
                 state->data.append(data);