Support non-enumerable `sql` and `values` in query object

[blakeembrey]

This issue come up in blakeembrey/sql-template-tag#2, based on the code in https://github.com/mysqljs/mysql/blob/master/lib/Connection.js#L39-L41 the query object copies properties verbatim from the input object. However, this is an issue for anyone wanting to have computed properties that are not enumerable (any get is not enumerable by default).

Happy if this will be denied, it's workaround-able, but it would be nicer to copy just the properties you need (e.g. ['sql', 'values', 'timeout']).