Added support for Timestamp and DocumentReference

[the0rem]

Closes #24

This PR is to add serialisation support for firestore.Timestamp and DocumentReference for use in makeDocumentSnapshot.

This fills in the gaps of functionality for the following:

• Online tests can now be written for Firestore functions that need to receive a DocumentReference as part of the DocumentSnapshot
• firestore.Timestamp can now be used instead of just Date. This is very useful when defining interfaces and models in TS where setting a timestamp had to use Date, but when getting the timestamp from the database would be firestore.Timestamp;

Tests are passing however I haven't added any new tests; There's currently no coverage for this part of the package.

[laurenzlong]

What was the test case you used to manually test this? Can you provide some sample code?

[laurenzlong]

Also, thank you for making this PR!

[the0rem]

I've simplified and obfuscated our code to produce the example however the method remains the same:

const test: FeaturesList = _test(appOptions, serviceKey)
const testFunction: WrappedFunction = test.wrap(someCloudFunction);
const subRecordRef: DocumentReference = admin.firestore().doc(appointmentPath);

const record: IRecord = mockRecord({
  ref: subRecordRef,
  date: Timestamp.fromDate(moment().toDate()),
});

const beforeSnap: DocumentSnapshot = test.firestore.makeDocumentSnapshot({}, path);
const afterSnap: DocumentSnapshot = test.firestore.makeDocumentSnapshot(record, path);
const change: Change<DocumentSnapshot> = test.makeChange(beforeSnap, afterSnap);

await testFunction(change, { params: testParams });

This PR changes are being used internally which is working a treat :)

[laurenzlong]

Sorry for the delay! I was at Firebase Summit. I made some suggestions to the code.

[the0rem]

Thanks for the feedback @laurenzlong. Those changes have been made and are passing our internal tests.

[the0rem]

Looks like travis failed for unrelated reasons.

[laurenzlong]

Looks good! Seems like Travis is failing because a dependency is failing for Node 11. I'll resolve that separately, before merging this PR. Thanks!

[the0rem]

Hi @laurenzlong any update on this?

[laurenzlong]

Sorry for the delay!

[laurenzlong]

This is now a part of v0.1.5