Add some documentation on using Mongomock to upgrade Pymongo.

Add default and options to get_default_database() (mongomock#693)

Add the `default=` parameter to `MongoClient.get_default_database()` to
equalize behavior with `pymongo`.

Forward remaining keyword arguments from
`MongoClient.get_default_database()` to `MongoClient.get_database()`.

Authored by leorochael

Fix type stubs for patch and test it to prevent regressions.

Fixes mongomock#691

Mark $convert and $toLong operators as not implemented yet.

For mongomock#685

Return the right document for find_one_and_replace using ReturnDocume…

…nt.AFTER and upsert.

Fix mongomock#671

Handling truthy and falsy values the way MongoDB does it (mongomock#651)

Fixes mongomock#650

Add an easy way to compare exception types between mongomock and pymo…

…ngo.

Implement the $facet operator. (mongomock#563)

Fixes mongomock#519.

Fix the version of MongoDB server installed for test.

Implement the $setUnion operator in the aggregate pipeline. (mongomoc…

…k#510)

Fixes mongomock#507.