Fixed keywordization.

[startling]

Previously *keywordize* was a dynamic variable, and so e.g. (map decode ...) would use the default *keywordize* (i.e. true) rather than the locally bound one, since map is the calling context. As you can imagine, this caused all kinds of flakiness when you wanted un-keywordized values anywhere but the top level in maps decoded from YAML.

I added a test case and then fixed it.