Change exception data member

from "reference to string" to "string" (Resolves the most serious part of issue #216)

Author: Dani-Hub

Diff for: src/lib_json/json_value.cpp

@@ -158,7 +158,7 @@ class JSON_API Exception : public std::exception {
   virtual ~Exception() throw();
   virtual char const* what() const throw();
 protected:
-  std::string const& msg_;
+  std::string const msg_;
 };
 class JSON_API RuntimeError : public Exception {
 public: