propagates promiseLibrary to new stub behaviors

[HugoMuller]

Purpose (TL;DR) - mandatory

Fix issue #1474 by copying stub promiseLibrary to its new behaviors.

Background (Problem in detail) - optional

The problem was that stub behaviors created with onCall, did not preserve the promise library defined when calling usingPromise on the original stub.

Solution - optional

Extending the new behavior with the promiseLibrary from the stub.defaultBehavior solves the issue.

How to verify - mandatory

1. Check out this branch (see github instructions below)
2. npm install
3. npm test

[coveralls]

Coverage increased (+0.004%) to 95.002% when pulling d14d296 on HugoMuller:issue-1474 into 9365d49 on sinonjs:master.

[mroderick]

Thank you for your contribution 💯

[mroderick]

This has become v2.3.8