Javascript objects are always serialized as BSON code types without scope

This bug dates back to the very first release and was introduced in dfbef38 for PHPC-74.

Javascript objects with a scope are erroneously serialized as BSON code types without a scope. Javascript objects without a scope are erroneously serialized as BSON code_with_scope types; however, since their scope pointer is null, bson_append_code_with_scope() defensively falls back to bson_append_code().

This was discovered while updating the PHPC-459 tests for libmongoc-1.5.0-rc6, which includes CDRIVER-1335.