Task: Prevent double json-decoding of attribute values

In bd4a7081, json-decoding of attribute values was introduced as a
workaround to properly handle blackslashes in description and
annotations (see TW-880).

However, this behaviour is no longer present with the new parser and
introduces its own suite of issues (i.e. see #2140).

Closes #2140.

src/Task.cpp

@@ -269,7 +269,7 @@ time_t Task::get_date (const std::string& name) const
 ////////////////////////////////////////////////////////////////////////////////
 void Task::set (const std::string& name, const std::string& value)
 {
-  data[name] = json::decode (value);
+  data[name] = value;
 
   if (! name.compare (0, 11, "annotation_", 11))
     ++annotation_count;