AsChangeset works on a struct that only consists of primary keys on nightly

[Drakulix]

Deriving AsChangeset works with the proc-macro code-generation, but not via syntex/macro generation, if fields turns out to be empty. This happens if the struct contains nothing but primary keys (e.g. n:m relation tables).

Having an AsChangeset implementation on such a struct is of course not very useful and should most likely not be supported. I just noticed this inconsistency and think it should be fixed.

[Eijebong]

This can be closed now that #1074 has landed.