Canonicalize non-GET URLs with native JSON values

[tw4l]

The way that pywb rewrites URLs for POST (and other non-GET) request canonicalization ends up writing Pythonic values into the URL such as True, False, and None, whereas we ideally want to have it use valid JSON values.

This is part of making POST canonicalization consistent across Webrecorder tools. Related to webrecorder/specs#141

[tw4l]

We will also need to ensure that the fuzzy matching in pywb and wabac.js works with previously-created URLs created by pwyb, or develop a process to reindex as necessary.