Issue 5013 - Make parameter const ref when applicable.

test/Prefs.cpp

 /*
  * 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 19 matching lines @@
     std::string prefsContents;
 
     std::shared_ptr<std::istream> Read(const std::string& path) const
     {
       if (path == "prefs.json" && !prefsContents.empty())
         return std::shared_ptr<std::istream>(new std::istringstream(prefsContents));
 
       return LazyFileSystem::Read(path);
     }
 
-    void Write(const std::string& path, const std::istream& content)
+    void Write(const std::string& path, std::istream& content)
     {
       if (path == "prefs.json")
       {
         std::stringstream ss;
         ss << content.rdbuf();
         prefsContents = ss.str();
       }
       else
         LazyFileSystem::Write(path, content);
     }
@@ skipping 157 matching lines @@
   {
     ResetJsEngine();
     AdblockPlus::FilterEngine::Prefs preconfiguredPrefs;
     preconfiguredPrefs["suppress_first_run_page"] = jsEngine->NewValue(true);
     auto filterEngine = CreateFilterEngine(preconfiguredPrefs);
 
     ASSERT_TRUE(filterEngine->GetPref("suppress_first_run_page")->IsBool());
     ASSERT_FALSE(filterEngine->GetPref("suppress_first_run_page")->AsBool());
   }
 }