The API request `add_field` method has had a deprecation warning for a while about passing in values that are not either `str` or `bytes`. This change replaces the warning+cast with an exception. There were a few places inside RBTools itself that were still hitting this, all in the post implementation. These have been fixed up to format the values as strings. Testing Done: - Ran unit tests. - Audited call sites to verify that we were passing string types everywhere. - Posted changes, including ones that needed to upload binary files.