Early in the development in the WebHook support, we'd have certain

objects slipping in (like model instances) that broke encoding, and this

made it unsafe to support custom payloads in a non-controlled

environment (such as RBCommons). We attempted to serialize everything to

a set of safe data types, and have now done so for a while.
This change ensures this going forward by taking the payload being

dispatched and normalizing it first, converting everything to a simple

list of allowed primitives (and datetime objects, which are useful in

templates and are safe). The normalization process traverses the payload

in much the same way that the API serialization code does, but in a way

that's a bit more safe and specific to the needs of WebHooks. In the

process, this checks for any types that aren't expected and, if found,

logs the failure and aborts the dispatch.
While we're not going to trigger these exceptions on our in production

today, this does ensure that we don't accidentally introduce something

that will trigger it in the future, and that extension authors don't

either.
Unit tests were updated to check the dispatch process and payload

content a bit more closely, ensuring that normalization and type

validation works correctly and testing more of the data sent in the

request. Much of this code was moved into a common helper to help

simplify tests. Other tests in the same file were also updated with

small fixes.

All unit tests pass.
Manually tested events with a WebHook destination, checking the payloads

to make sure they contained what I expected and were received.