Change return type on File::asZString.

File.h

@@ -25,7 +25,7 @@ namespace coreutils {
       void write(std::string data);
       void write(coreutils::ZString &data);
       std::string asString();
-      coreutils::ZString& asZString();
+      coreutils::ZString asZString();
       char *buffer;
       size_t size;
       bool eof();