Fix call site when using Pool.prototype.query

This fixes the call site for long stack traces when using the `query` method on a pool object. Before the stack trace would only lead up to inside the `getConnection` method instead of the library caller. This results in the following change:

```javascript
var mysql = require('mysql');
var pool = mysql.createPool(...);

pool.query('ERROR', function (err, rows) { // 1
  if (err) throw err;
  // The stack trace now includes the line marked 1
  // Prior to this, the stack trace included nothing from this file
});
```