fixes.
- Fixed bugged logic in _STIXBase._check_at_least_one_property(),
and revamped the code to be simpler and clearer.
- Changed custom extension registration to auto-create an
"extension_type" property based on the attribute of that
name on the custom class, if present.
- The custom extension registration change above uncovered
what seemed like a bug in a unit test: a custom extension
was registered, but it was not given an extension type. The
test used the extension as extension_type="property-extension";
this now causes a standard error about an extra property. I
fixed the test to assign the custom extension the proper type.
d87718c15c