Add hasOwnProperty checks where appropriate

For boolean-like objects, I've added hasOwnProperty checks in addition to the existing truthiness check even though for most of these dicts, we leave out false keys instead of setting them explicitly to false.

In DOMPropertyOperations, we don't need to check hasOwnProperty if the property is in isStandardName because (with the exception of getPossibleStandardName) the dicts on DOMProperty will now be consistently populated with every valid attribute.