Fix serval bugs, such compile error & safe string assignment

I wonder to know why you delete copy ctor & move ctor in MySqlException ? and it cause a compile error below.

```
In file included from MySql.hpp:19:0,
                 from examples.cpp:1:
OutputBinder.hpp: In static member function ‘static void OutputBinderPrivate::OutputBinderResultSetter<signed char>::setResult(int8_t*, const MYSQL_BIND&)’:
OutputBinder.hpp:311:62: error: use of deleted function ‘MySqlException::MySqlException(const MySqlException&)’
                 throw MySqlException(NULL_VALUE_ERROR_MESSAGE); \
```

Thanks for you work.