Issue 5013 - Make parameter const ref when applicable.

include/AdblockPlus/FileSystem.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 63 matching lines @@
      */
     virtual std::shared_ptr<std::istream>
       Read(const std::string& path) const = 0;
 
     /**
      * Writes to a file.
      * @param path File path.
      * @param data Input stream with the data to write.
      */
     virtual void Write(const std::string& path,
-                       const std::istream& data) = 0;
+                       std::istream& data) = 0;
 
     /**
      * Moves a file (i.e.\ renames it).
      * @param fromPath Current path to the file.
      * @param toPath New path to the file.
      */
     virtual void Move(const std::string& fromPath,
                       const std::string& toPath) = 0;
 
     /**
@@ skipping 17 matching lines @@
     virtual std::string Resolve(const std::string& path) const = 0;
   };
 
   /**
    * Shared smart pointer to a `FileSystem` instance.
    */
   typedef std::shared_ptr<FileSystem> FileSystemPtr;
 }
 
 #endif