fix: avoid fake .then() method on mysql2 Query class

[mickdekkers]

Issue #, if available: #452

Description of changes:

The mysql2 package has a Query class with a .then() method which throws an error when called.
This confuses the current patching code, which sees this object as a Promise and calls .then() on it.

As a result, code like this currently breaks with an error:

const captureMySQL = require('aws-xray-sdk-mysql');
const {createConnection} = captureMySQL(require('mysql2'));

const connection = createConnection(someConfig);
const queryStream = connection
        .query(someSql) // <-- returns Query class, error is thrown here
        .stream({highWaterMark: 10});

// (Attempt to) do stuff with queryStream
queryStream.pipe(someFileOnDisk);

This PR fixes this unfortunate interaction by performing an additional check to determine whether a returned value is actually a Promise.

Beyond the new unit test (which fails without the change and passes with it), I can confirm this change has fixed the error in our application.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Fixes: #452

[willarmiros]

Thanks for this fix!