Don't hardcode fields when getting or updating subject

Currently the API call to update a subject [hardcodes the fields](https://github.com/WordPress/try-wordpress/pull/160/files#diff-9b98c46d7425255749cff966d7ae4ad47becbb5e1a510cd16577b5dcb7553c0cR79-R101). This can only be addressed once #138 has been fully tackled.