Cleanup: Don't use string literals when character literals are needed

src/Msg.cpp

@@ -72,9 +72,9 @@ std::string Msg::serialize () const
 {
   std::string output;
   for (auto& i : _header)
-    output += i.first + ": " + i.second + "\n";
+    output += i.first + ": " + i.second + '\n';
 
-  output += "\n" + _payload + "\n";
+  output += '\n' + _payload + '\n';
   return output;
 }
 
@@ -95,7 +95,7 @@ bool Msg::parse (const std::string& input)
   {
     auto delimiter = i.find (':');
     if (delimiter == std::string::npos)
-      throw std::string ("ERROR: Malformed message header '") + i + "'";
+      throw std::string ("ERROR: Malformed message header '") + i + '\'';
 
     _header[Lexer::trim (i.substr (0, delimiter))] = Lexer::trim (i.substr (delimiter + 1));
   }