Fix parsing of bookmark objects

[ndhansen]

What is this MR?

This MR fixes the bug described in #363, and includes a test to show it's working. It's arguably a breaking change, because bookmark objects will no longer contain a parsed object fields, but I would argue that this is fine because the parsing didn't work anyway, so it's unlikely anyone was relying on this.

There are other ways of fixing this, such as handling bookmarks early and short-circuiting, but I decided to go for more minimal changes to the unmarshal logic.

How is this tested?

I added a test which includes a real payload from a bookmark event obtained when watching a kubernetes namespace, which previously would cause the client to crash.

I tested this manually by running the bookmark_bug.py script described in the issue, and got the expected output:

$ python streaming_test.py
ADDED
BOOKMARK
BOOKMARK
...