Fix JsonDocument::clear() does not clear the KEY_IS_OWNED flag in VariantData's _flags variable

Created by: DanielEbert

If JsonDocument::clear() is called, the KEY_IS_OWNED flag of JsonDocument's _data variable is not cleared.

Currently, JsonDocument::clear() calls _data.setNull, which calls setType(VALUE_IS_NULL), and setType preserves the KEY_IS_OWNED flag due to the '_flags &= KEY_IS_OWNED;' statement.

In this pull request, the _data.init() call was added to the JsonDocument::clear() method so that the KEY_IS_OWNED flag is cleared.